00:01
We have a question here, it's a double integral.
00:04
0 to pi d theta, 0 to cos theta, d r, and the integrand is sine theta times r.
00:16
So, when we do the r integral, this is, this r is going to turn into a half r squared, and we're evaluating that between 0 and cos theta.
00:34
At zero, it's going to give nothing, so this is just a half, 0 to pi, d theta, sine theta, cos squared theta.
00:46
Now if we think about it, the derivative of cos cubed theta is going to give us three times sine theta, because that's the derivative of cos theta, times cos squared theta.
00:58
Because we're just using the chain rule here.
01:01
So if we multiply by a third, cos cubed theta, and we have a minus sign here as well, because the derivative of cause is minus sign, if you take the derivative of this, you're going to find that this is what it is...