Trigonometric differentiation question

Hey guys, I’m working on this question here

The gradient of the curve y=cos2x when x=15 is:

The options are

-pi/180
-1
pi(sqrt(3))
1

I differentiated it, substituted 15 in and I got -1. For some reason it’s saying this is incorrect but I don’t see how. The question specifies that this is in degrees and not radians btw.

Reply 1

DFranklin

18

Original post by ramell

Hey guys, I’m working on this question here

The gradient of the curve y=cos2x when x=15 is:

The options are

-pi/180
-1
pi(sqrt(3))
1

I differentiated it, substituted 15 in and I got -1. For some reason it’s saying this is incorrect but I don’t see how. The question specifies that this is in degrees and not radians btw.

When you do calculus with trig, you always work in radians (i.e. the argument to sin, cos etc must be in radians). That means you need to convert your argument from degrees to radians.

Reply 2

ramell

OP

2

Original post by DFranklin

When you do calculus with trig, you always work in radians (i.e. the argument to sin, cos etc must be in radians). That means you need to convert your argument from degrees to radians.