Derivatives of Trigonometric Functions

[DMac]

[SOLVED] Derivatives of Trigonometric Functions

I need to find the critical numbers of this function:
y = cos x - sin x where -pi <= x <= pi

I found the derivative as:

dy/dx = -(sin x + cos x)

But when I equate dy/dx to zero, I get:

sin x + cos x = 0...where do I go from here?

 

[nicksauce]

Well when does sinx = -cosx?

 

[DMac]

Ha, lolz I can't believe I didn't think of tan x = -1. (It's getting late, and I've only had 5 hours of sleep these past two nights.) Thanks for the help. =D