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# Core 4 question watch

1. Right well im stuck on a volume of revolution question, i have to integrate ((10cos(x-alpha))^2 but i dont know how to actually square that.
Apparently the answer of ((10cos(x-alpha))^2 is 100*0.5(1+cos(2x-2alpha))

any one able to show me how to get to that please?
2. Think about what you get if you do (2x)^2
Apply the same to (10cos(x-a))^2
3. (Original post by Ben121)
Think about what you get if you do (2x)^2
Apply the same to (10cos(x-a))^2
That does not help!!
4. It should do
(2x)^2 is 4x^2. You square both parts inside.
Doing that with your expression gives you (100 * cos(x-a)^2)
You can then use an identity to rewrite cos(x-a)^a and integrate.
5. (Original post by Ben121)
It should do
(2x)^2 is 4x^2. You square both parts inside.
Doing that with your expression gives you (100 * cos(x-a)^2)
You can then use an identity to rewrite cos(x-a)^a and integrate.
Ow yeah sorry i get that, my question should of been i dont get which identity to use then.
6. (Original post by Sheldon)
Right well im stuck on a volume of revolution question, i have to integrate ((10cos(x-alpha))^2 but i dont know how to actually square that.
Apparently the answer of ((10cos(x-alpha))^2 is 100*0.5(1+cos(2x-2alpha))

any one able to show me how to get to that please?
you've put 3 open brackets and only 2 close brackets. ??
7. (cos x)^2 = 0.5(1 + cos(2x))

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