00:03
Okay, this question asks for the slope of a tangent line to a polar curve, and i'm at a certain point theta equals pi over 4.
00:17
A quick sketch of the graph is not necessary for the problem, but i'm going to go ahead and show you what it looks like, just so we know.
00:24
When theta is 0, sine is 0, and i get r is 1.
00:30
When theta is pi over 2, i get 1, so i go out 2 spaces, r is 2.
00:37
At pi, i get 1, and at 3 pi over 2, i get 0.
00:43
This is a rough sketch of a cardioid.
00:48
And they want me to look at an angle of pi over 4 and find the slope of the tangent line.
00:53
I'm expecting a negative number.
00:56
Okay, slopes of tangent lines work like this.
00:59
I need dy, d theta, divided by dx, d theta.
01:05
I don't have y and x yet, but i'm going to use x.
01:08
X is r cosine theta, and y is our sine theta.
01:16
So dx, d theta is a product rule.
01:19
It'll be r prime cosine theta plus r.
01:26
Sorry, it'll be minus r because that's the derivative of cosine.
01:31
The derivative r times cosine plus r times the derivative of cosine, which is negative sign.
01:39
D .y d theta is r prime sine theta plus r times the derivative of sign which is cosine theta...