WEBVTT
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In this video, weโre gonna use the algebraic process of elimination to solve two simultaneous linear equations.
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Weโll go through to some examples and weโll built from simple questions to more complicated ones that require a bit more manipulation, and have even fractional or negative or even zero solutions.
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So example one: Use algebraic elimination to solve these simultaneous equations.
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And weโve got ๐ plus three ๐ equals seven and ๐ plus ๐ equals three.
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So weโve got two separate equations and theyโre linear equations cause they havenโt got any ๐ squareds, or ๐ squareds, or ๐ cubes, or ๐ cubes, or anything like that.
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And each individual equation has an infinite number of solutions.
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So for example, if we took the first one ๐ plus three ๐ equals seven, if I put ๐ equal to one and ๐ equal to two, Iโd have one plus three lots of two, one plus six equals seven, so that would work.
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I could put ๐ equal to four and ๐ equal to one and that would work.
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Or even ๐ equal to zero and ๐ equal to seven.
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I could put ๐ equal to eight and then ๐ equal to negative a third and that would work as well.
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In fact, I could put any value in for ๐ and I could generate a version of ๐ which would make that equation work.
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So as I say, weโve got an infinite number of solutions to this.
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And likewise, Iโve got an infinite number of solutions to the other equation ๐ plus ๐ equals three.
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Whatever I- whatever value I put in for ๐, I can come up with a value of ๐ which would make their sum equal to three.
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For example, ๐ is a hundred, ๐ is negative ninety-seven; add them together you get three.
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And this is what happens if youโve got one equation with two unknowns in it, then there are gonna be an infinite number of combinations of those two variables which will make that equation true.
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Now what weโre looking for here on these simultaneous equations, both of these equations have to be true.
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So weโre looking for a pair of ๐- and ๐-values which will be true in this one, and the same pair of ๐- and ๐-values will be true in this one.
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And there may be one solution; there may be two solutions, who knows.
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In this particular case, I think weโll find thereโs only one solution.
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So before we go and solve this algebraically, letโs just take a look at the graphs of these and then-then Iโll tell you why I think there was only one solution.
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So using the ๐ฅ-axis as our ๐-axis and the ๐ฆ-axis as our ๐-axis, for example weโve got ๐ plus ๐ equals three here.
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Now I could put any value I like in for ๐.
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I could put a value of four in for ๐.
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And if I read off the corresponding coordinate, Iโve got a-a ๐-value of negative one.
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I could have an ๐-value of negative one and that would generate a ๐-value of four.
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And that works for the other equation.
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So ๐ plus three ๐ equals seven.
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So if I put in an ๐-value of five, my ๐-value, if I read this off, is about nought point six and a bit.
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If I put in an ๐-value of negative two, reading off my ๐-value, that looks like itโs gonna be three.
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So I think that you see thereโre an infinite number of pairs of coordinates, if you like, along each of those lines.
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They give us the solutions that match those particular equation, so a whole bunch of different pairs of ๐- and ๐-values along the blue lines- the blue line, which matches that equation.
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But thereโs one point here where those lines crossover, those straight lines crossover, where the ๐ and ๐ combinations will work on the blue line and they will also work on the red line.
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Thatโs what weโre looking for in this simultaneous equation.
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Now in this particular case, it looks like weโre gonna get an ๐-value of one and a ๐-value of two.
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And thatโs probably gonna be our exact answer.
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But of course itโs not always gonna be exact; itโs not always possible to read exact answers on our graphs.
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So letโs forget that we saw the graph and now more look at this algebraic method, which is hopefully always gonna give us a hundred percent accurate answer.
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Now when I do simultaneous equation questions, I always do like a little curly brace like this, to indicate the fact that theyโre simultaneously true.
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You donโt have to do that, not all places do that.
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But I quite like that method and I always number my equations so that I can give a running commentary of what Iโm doing in my working out, and refer to those numbers.
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So youโll see what that looks like as we go through.
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Now the way these equations are laid out, Iโve got ๐ plus three ๐ is equal to seven in the first one, and Iโve got ๐ plus one ๐ is equal to three in the second one.
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So theyโre sort of laid out so that the ๐s are above each other, the ๐s are above each other, and then the numbers on the other side of the equation are above each other.
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So what I could do is, I could take equation one and then I could subtract equation two from that, term by term.
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So letโs have a look at what that would look like.
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So equation one take away equation two, so Iโm gonna have ๐ take away ๐ in the first column; so ๐ take away ๐.
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And then Iโm gonna have positive three ๐ take away ๐ in the second column; so positive three ๐ take away ๐.
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And on the right-hand side, Iโm gonna have seven take away three.
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Now ๐ take away ๐ is nothing, so that process eliminated ๐.
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So weโve created a new equation here and we could call it equation three, which has eliminated ๐.
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Because we had the same number of ๐s in each, if I take one away from the other Iโve got no ๐s left.
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Three ๐ minus ๐ is two ๐, and seven take away three is four.
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So doing equation one and subtracting equation two from it has eliminated ๐ and left us with a new equation, which weโll call equation number three; two ๐ is equal to four.
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Now if I divide both sides of that equation by two, a half of two ๐ is one ๐, so just ๐ and a half of four is two.
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So we now know that ๐ is equal to two.
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So now I could substitute that value of ๐ into either equation two or equation one, to find out what the corresponding ๐-value would be.
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Now Iโm gonna substitute it into equation two because the numbers look a bit smaller.
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Itโs just ๐ plus ๐ equals three.
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So itโs just a small advantage, but it ma- itโs gonna make the math a little bit easier.
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So substituting the value ๐ equals two into equation two.
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Equation two begins with ๐ plus ๐.
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Well we know that ๐ equals two so instead of writing two, Iโm just gonna- instead of writing ๐, Iโm just gonna write two.
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And thatโs equal to three, so ๐ plus two equals three.
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Now Iโve got an equation ๐ plus two equals three.
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I can take away two from both sides of that equation.
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So on the left-hand side, ๐ plus two take away two is just ๐ and on the right-hand side, three take away two is one.
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So Iโve got ๐ equals one.
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Well thatโs great cause that tailored with the graph that we saw earlier, but you gotta remember we wouldnโt normally have seen the graph by now.
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So we need to have a way of just checking that our answer is correct.
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And we used equation two to do some substituting of ๐, so weโve got equation one that we havenโt really played around with yet.
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So Iโm gonna substitute the values ๐ and ๐, so ๐ is one, ๐ is two, back into equation one now just to do a check.
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So the value of ๐ is one and then itโs plus three ๐, so thatโs plus three times the value of ๐ and the value of ๐ is two.
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And weโre hoping that thatโs gonna equal seven.
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Well one and three times two is six, one plus six does in fact equal seven.
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So yep, weโre happy that thatโs the right answer.
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So all that remains is to put that answer there in a nice box and make it lovely and clear.
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Okay.
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Letโs go on to the next question.
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And now weโve gotta use algebraic elimination again to solve the simultaneous equations three ๐ plus two ๐ equals seventeen and four ๐ minus two ๐ is equal to four.
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So Iโm gonna put my braces around them to show theyโre simultaneous and Iโm gonna number them one and two.
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Now if I do equation one take away equation two, Iโm gonna have three ๐ take away four ๐, which is gonna give me negative ๐.
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So Iโm not gonna eliminate the ๐s.
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And if I did two ๐ take away negative two ๐, thatโs the same as two ๐ add two ๐, thatโll be four ๐.
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So Iโm not gonna eliminate ๐.
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So Iโm just gonna come up with another equation with two variables in it.
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So thatโs not gonna work.
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But looking at these equations, Iโve got the same number of ๐s.
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There are two ๐s in each, but in one case itโs positive two ๐, the other case itโs negative two ๐.
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So if I add the two equations together, Iโm gonna get two ๐ plus negative two ๐.
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Thatโs gonna be no ๐.
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So thatโs gonna cancel out the ๐s.
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So thatโs what Iโm gonna do.
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So three ๐ plus four ๐ is seven ๐, two ๐ add negative two ๐ is nothing, and seventeen plus four is twenty-one.
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So seven ๐ is equal to twenty-one.
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I want to know what one ๐ is, so if I divide both sides by seven, I get ๐ is equal to twenty-one divided by seven is three; ๐ is equal to three.
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So Iโm now gonna substitute that back into either equation, one or two, in order to work out the value of ๐.
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So Iโm gonna go for equation one.
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Just for the sake of argument, it doesnโt really matter which way around you go.
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So ๐ is three.
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So that first term there is three times three plus two ๐ is equal to seventeen, so thatโs nine plus two ๐ is equal to seventeen.
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So if I take away nine from each side of that equation, Iโll just leave the ๐ term on itโs own on the left.
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So as I say nine take away nine leaves me no number, so thatโs just the two ๐.
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And seventeen take away nine leaves eight.
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So now I can just divide both sides by two, and Iโve got ๐ equals four.
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So thereโre my answers: ๐ is three and ๐ is four.
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Now I used equation one to do my substitution for ๐, so Iโm gonna use equation two to do my check now.
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So all being well, four ๐, so thatโs four times three minus two ๐; so thatโs two times four is equal to four.
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Well four times three is twelve and two times four is eight.
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And twelve minus eight does in fact equal four.
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So weโre pretty confident weโve got the right answer.
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Number three then: Use algebraic elimination to solve the simultaneous equations two ๐ plus four ๐ equals ten and four ๐ minus ๐ equals eleven.
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So letโs label up our equations, one and two.
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Now when we look at the terms, Iโve got two ๐ in the first and four ๐ in the second; Iโve got four ๐ in the first and negative ๐ in the second.
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If I just add or subtract equations one or two, Iโm not gonna eliminate either of those variables.
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Iโm gonna, you know, end up with some ๐s and some ๐s in my equation.
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So what I need to do is generate a new equation thatโs either got the same number of ๐s or the same number of ๐s, as one of the other equations.
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Now if multiply everything in the first equation by two, Iโll have four ๐ plus eight ๐ equals twenty.
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But Iโll have four ๐, so Iโll have the same number of ๐s as I did in equation two.
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So letโs call that equation three.
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Now youโll also notice that I couldโve multiplied everything in equation two by four and that wouldโve given me sixteen ๐, which is different.
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But it wouldโve given me negative four ๐, so that wouldโve worked just as well.
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But Iโd rather multiply everything by two than multiply everything by four, just to get the numbers smaller.
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So thatโs what weโve done.
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So equations two and three have now got the same number of ๐s; they both got four ๐s.
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So Iโm gonna take one equation away from the other to eliminate the ๐s.
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Now I could either do equation two take away equation three, or I could do equation three take away equation two.
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Both of those would eliminate the ๐s.
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But if I do three minus two, then Iโm gonna have eight take away negative ๐, so Iโm gonna have nine ๐.
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If I did it the other way around, Iโd have negative ๐ take away eight, so Iโd have negative nine ๐.
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And Iโd rather end up with a positive number of ๐s on one side of my equation, cause that just makes subsequent steps a little bit easier.
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So thatโs what weโre gonna do.
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Equation three take away equation two.
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So as we say four ๐ minus four ๐ is nothing, eight ๐ take away negative ๐ is positive nine ๐, and twenty take away eleven is nine; so weโve got nine ๐ is equal to nine.
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Now I can divide both sides by nine, so that I get the result of ๐ is equal to one.
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And I can substitute that back into equation one, two, or three in order to find out the next bit of the answer, to find out the value of ๐.
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Now Iโm gonna substitute it into equation two, which is gonna give me four ๐ take away one is equal to eleven.
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And if I add one to both sides of that equation, four ๐ take away one plus one is just four ๐ and on the right-hand side, eleven plus one is twelve.
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Now dividing both sides by four, I get ๐ equals three.
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And I used equation two so Iโm gonna check my answers in equation one.
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So substituting in three for ๐ and one for ๐, Iโm hoping that two times three plus four times one equals ten.
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Well two times three is six and four times one is four and six plus four is ten.
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So hopefully youโve got the right answer.
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So number four: Use algebraic elimination again to solve the simultaneous equations three ๐ minus five ๐ equals fifteen and two ๐ plus seven ๐ equals forty-one.
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Well I canโt simply subtract one from two or two from one to eliminate ๐ or ๐ and Iโm- neither of them are simple multiples of the other.
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So I canโt just multiply one equation by something to get the same number of ๐s or ๐s as the other one, well not whole numbers anyway.
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So what if I multiply the first equation by two and the second equation by three?
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Well doubling everything in the first equation three ๐ times two is six ๐, minus five ๐ times two is minus ten ๐, and fifteen times two is thirty.
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And three times the second equation gives us six ๐ plus twenty-one ๐ is equal to a hundred and twenty-three.
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Now letโs call those equations three and four.
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So now Iโve got the same number of ๐s in each, and Iโm gonna be able to subtract one of those equations three and four from the other, in order to eliminate the number of ๐s.
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Now just before we go on, just to bear in mind, we couldโve multiplied equation one by seven and equation two by five, and then we weโd have had thirty-five ๐s in each case.
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And we couldโve eliminated ๐.
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But I thought it was easier to multiply it by two and three, than it is to multiply it by five and seven cause it will leave us with slightly smaller numbers.
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So are we gonna do equation four subtract equation three?
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Or are we gonna do equation three subtract equation four?
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Well my top tip will be four minus three.
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Because that means weโre gonna do six ๐ take away six ๐ is nothing, and twenty-one ๐ take away negative ten ๐ is the same as adding ten ๐ so thatโs gonna be thirty-one ๐, and a hundred and twenty-three minus thirty is ninety-three.
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So weโve ended up with a positive number of ๐s and a positive number on the right-hand side, so thatโs gonna make life a little bit easier.
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Now I just need to divide both sides by thirty-one.
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And for those of you who are not so comfortable with your thirty-one times table, it- very often youโll find in these questions that the numbers workout nice and easily.
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Three times thirty-one is ninety-three, so we end up with ๐ equals three.
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Now Iโm gonna substitute that value for ๐ into equation one.
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Now I couldโve substituted it into equation two just as happily, but the numbers looked a bit smaller in equation one; so I went for that one.
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So weโve got three ๐ minus five lots of ๐ is three, so five lots of three is equal to fifteen.
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So just evaluating that five times three is fifteen so three ๐ minus fifteen is equal to fifteen.
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If I add fifteen to both sides, Iโll eliminate the negative fifteen from the left-hand side.
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So thatโs gives us three ๐ is equal to thirty.
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And then dividing both sides by three, I get ๐ equals ten.
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So letโs just quickly do a check.
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We used equation one to substitute, remember.
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So letโs use equation two to do our check.
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And itโs two ๐, so thatโs two times ten plus seven ๐, so thatโs seven times three.
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And weโre hoping that equals forty-one.
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And indeed twenty plus twenty-one does equal forty-one.
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So weโve got our answer ๐ equals ten and ๐ equals three.
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Now number five, Iโm not gonna go through in great detail.
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Weโve got four ๐ minus two ๐ equals eight and six ๐ plus four ๐ equals negative nine.
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So pause the video and have a good look, if you want to, to see all the working out.
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But I just wanted to highlight the fact that sometimes the answers can be fractions and sometimes the answers can be negative.
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And sometimes the answer can even be zero, as in this question here.
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And thatโs something that often catches people out.
00:17:12.010 --> 00:17:20.460
So just one more thing, sometimes you need to do a little bit of manipulation to get your ๐s and ๐s to line up properly in the first place, so like in this example.
00:17:21.830 --> 00:17:30.710
So Iโve done lots of rearranging just to get two equations with the same number of ๐s, and all the ๐s and the ๐s and the other numbers lined up properly.
00:17:32.260 --> 00:17:38.200
So we can then subtract some equations to eliminate ๐ and go on and solve the problem.