# Thread: This is a problem with trig involved and pi

1. ## This is a problem with trig involved and pi

The problem is cos(-π/4+401π) is equal to (blank), if you don't understand, cos(-pi/4+401pi). I think the first step is cos(-π/4+(2x200)π+1)

2. ## Re: This is a problem with trig involved and pi

Hey OfficeSnail.

What is the rule about cos(x + pi) and cos(x + 2*pi) and how can you get cos(x + 400*pi + pi) in terms of cos(x) or sin(x)?

3. ## Re: This is a problem with trig involved and pi

Look office snail, you can write $cos(401\pi + \frac{-\pi}{4}) = cos(400\pi + \frac{3\pi}{4}) = cos(2\pi*200 + \frac{3\pi}{4})$.