WEBVTT
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A cylinder-shaped jar of jam has a circular base of radius seven.
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Use twenty two over seven as an approximation of đťś‹ to calculate the perimeter of the base.
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Okay, letâ€™s draw a quick sketch of our jar of jam.
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Itâ€™s a cylinder shape, so there it is.
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And weâ€™re told that it has a circular base of radius seven units.
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Weâ€™re not told what those units are so we wonâ€™t be able to write those down, but seven units.
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So the radius is the distance from the centre of a circle to its circumference.
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Now weâ€™re trying to work out the length of the perimeter of the base.
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Weâ€™re told the base is a circle so we want to know the perimeter of a circle radius seven.
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Now weâ€™ve got two possible formulae that we could work with.
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We could do đťś‹ times the diameter of the circle or we could do two times đťś‹ times the radius of the circle.
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Now weâ€™ve been given the radius in this case, so weâ€™re gonna use the second of those formulae.
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So thatâ€™s two times, and then we were told to use twenty-two over seven as an approximation of đťś‹, so weâ€™re gonna write two times twenty-two over seven.
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And we were told that the radius was seven.
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So our calculation becomes two times twenty-two over seven times seven.
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Now Iâ€™m gonna turn this into a proper fraction calculation.
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But instead of writing two, Iâ€™m gonna write two over one.
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And instead of writing seven, Iâ€™m gonna write seven over one.
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And now when weâ€™re multiplying fractions together, I can just multiply all the tops together and then all the bottoms together, all the numerators together and then all the denominators together.
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But before I do that, I can see that Iâ€™ve got a seven as one of the numerators but Iâ€™ve also got a seven as one of the denominators, so I can do some cancelling.
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If I do seven divided by seven, I get one.
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If I do seven divided by seven, I also get one.
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So Iâ€™ve got two over one times twenty-two over one times one over one.
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Well two times twenty-two times one is forty-four, and one times one times one is one.
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Well forty-four over one is just the same as forty-four.
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And we werenâ€™t told specifically what the units were, so Iâ€™m just gonna write forty-four units.
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So there we have our answer: forty-four units.