00:01
So what we're trying to do here is derive equations 1 when we have a theta value between pi over 2 and pi.
00:13
And we know that the circle rotated by an angle theta has a center of r theta, and the y coordinate of the circle will be r because the radius is r, and it's in contact with the x axis.
00:35
At theta equals 0, we know that the x coordinate will ultimately just be 0.
00:40
So the circle is always in touch with the x -axis.
00:44
So we know that the distance moved by some point p will always be the x -coordinate of the center.
00:54
So one way that we can show that is that the x -coordinate of the center will be the arc length rotated, which is exactly this right here, r -times theta.
01:06
So since we'll call a .r for arc length, that's going to be the same thing as r -theta r.
01:17
And we will let alpha be theta.
01:23
So we'll let alpha be equal to theta minus pi over two.
01:31
So when we do that, we can have a circle...