WEBVTT
00:00:00.720 --> 00:00:01.960
Here is a triangle.
00:00:02.720 --> 00:00:05.880
Measure the shortest side accurately, in centimetres.
00:00:06.560 --> 00:00:09.800
Measure the largest angle.
00:00:09.800 --> 00:00:15.440
Now looking at this triangle, this angle here is greater than 90 degrees.
00:00:16.120 --> 00:00:18.960
These two angles are smaller than 90 degrees.
00:00:19.560 --> 00:00:23.680
So this angle is the largest angle.
00:00:23.920 --> 00:00:29.600
And opposite the largest angle, you have the longest side.
00:00:29.920 --> 00:00:32.280
Now we want to measure the shortest side.
00:00:33.560 --> 00:00:37.280
So we definitely don’t want to measure the length of the longest side.
00:00:38.480 --> 00:00:42.160
Now just looking at the two shorter sides, they look roughly the same length.
00:00:42.680 --> 00:00:46.800
We’re gonna have to measure both of them in order to see which one is the shortest.
00:00:48.200 --> 00:00:53.880
So let’s measure the length of this side, this side, and this largest angle.
00:00:55.480 --> 00:01:04.960
Carefully lay down your ruler so that it’s parallel; it lines up with one of those lines and the zero mark here lines up perfectly with that corner.
00:01:06.560 --> 00:01:09.920
Then, we can see where the other end of the line goes up to.
00:01:11.760 --> 00:01:24.720
Our ruler measures in centimetres: one centimetre, two centimetres, three centimetres, four centimetres, five centimetres, six centimetres, seven centimetres.
00:01:24.720 --> 00:01:29.200
And these longer lines here tell us where the whole centimetre markings are.
00:01:30.440 --> 00:01:34.320
Now the space between each whole centimetre is split into 10.
00:01:34.880 --> 00:01:38.640
So each of those little lines represent a tenth of a centimetre.
00:01:40.000 --> 00:01:47.960
Looking at our baseline, it goes from zero on the scale up to six whole centimetres and another seven tenths of a centimetre.
00:01:49.120 --> 00:01:51.880
So that makes 6.7 centimetres.
00:01:52.280 --> 00:01:56.000
The baseline of our triangle is 6.7 centimetres long.
00:01:57.520 --> 00:02:00.840
Now let’s use the same method to measure this other short side.
00:02:01.360 --> 00:02:05.520
Carefully line the ruler up with the line with the zero mark at one corner.
00:02:06.040 --> 00:02:09.680
Then, we need to look carefully where does the next corner line up.
00:02:11.240 --> 00:02:15.440
It’s five whole centimetres and seven tenths of a centimetre.
00:02:15.880 --> 00:02:18.200
So that’s 5.7 centimetres.
00:02:19.480 --> 00:02:23.200
So the shortest side is 5.7 centimetres.
00:02:25.200 --> 00:02:30.240
Now we’ve measured the shortest side, let’s go on to measure the size of the largest angle.
00:02:32.040 --> 00:02:34.360
To measure angles, we need a protractor.
00:02:36.000 --> 00:02:39.480
Find the baseline between the zeros on your protractor.
00:02:41.520 --> 00:02:45.680
Make sure you line it up accurately with one of the sides of your triangle.
00:02:47.520 --> 00:02:52.680
Look carefully for the point on the baseline of your protractor, where the 90-degree line overlaps it.
00:02:53.640 --> 00:02:59.400
Carefully line up that point with the corner of the triangle that makes the angle you’re trying to measure.
00:03:00.920 --> 00:03:10.920
Think of this as a target: your target has to line up perfectly with the corner and the baseline of your protractor has to perfectly line up with one of the sides of the triangle.
00:03:13.000 --> 00:03:17.120
Notice that you got two sets of numbers going round the outside of your protractor.
00:03:18.880 --> 00:03:25.080
We’re going to use the scale that starts at zero on the side of the triangle where we’ve lined up our protractor.
00:03:26.640 --> 00:03:33.240
Now remember it’s this angle here between the two sides of the triangle that we’re trying to measure.
00:03:33.240 --> 00:03:41.040
So we’re gonna start counting at zero and work our way round that scale until we hit the other side of the triangle.
00:03:41.720 --> 00:03:57.680
In this case, we’re going to count through 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, and then a little bit.
00:03:59.080 --> 00:04:03.920
So our largest angle is 140 degrees plus some more degrees.
00:04:05.160 --> 00:04:10.440
Now you’ll notice between every 10 degrees, there are 10 little marks or 10 parts.
00:04:10.840 --> 00:04:13.720
So each one of those little parts represents one degree.
00:04:15.320 --> 00:04:21.600
If we count the little parts carefully between 140 and where our line is, there are five little parts there.
00:04:21.600 --> 00:04:23.720
So that represents five degrees.
00:04:25.240 --> 00:04:31.640
So our angle measures 140 degrees plus five degrees which is 145 degrees.
00:04:33.120 --> 00:04:39.880
Now this makes sense because our angle is clearly bigger than 90 degrees, but not as big as 180 degrees.
00:04:41.200 --> 00:04:46.040
If we’d have used the other scale, we might have said it was somewhere around 40 degrees.
00:04:46.480 --> 00:04:49.440
And that’s clearly wrong because it’s way less than 90 degrees.
00:04:51.360 --> 00:04:58.160
So it’s useful to line up your protractor and then just check at the end: does it make sense that it’s roughly that size?
00:04:58.440 --> 00:05:02.240
Now just before we go, I’m gonna show you another way of measuring that angle.
00:05:03.840 --> 00:05:07.680
We could have lined up the baseline of our protractor with the other side of the triangle.
00:05:08.800 --> 00:05:13.800
We’d still need to carefully line up the crosshair of the protractor with this corner of our triangle.
00:05:15.160 --> 00:05:19.000
But this time, we’d have started counting from zero on the outer scale.
00:05:20.480 --> 00:05:26.800
But again, we’d have counted up to 140 and then an extra five degrees but in the other direction.
00:05:27.520 --> 00:05:32.320
So either way, the largest angle is 145 degrees.