# Use trig identities to prove

1. Sep 17, 2007

$$\cos (t)=\cos(t+2\pi)$$

I know it is kind of silly, but I need to do it.

I could have sworn that a sum formula would have worked, ie.,
$$\cos(\alpha+\beta)=\cos \alpha \cos \beta -\sin \alpha \sin \beta$$

but when I sub in $$t$$ and$$2\pi$$ for the RHS, I am getting $$-\cos t$$

Where did I go wrong....

Casey

Edit: IDK why LaTex alpha isn't showing on RHS

Last edited: Sep 17, 2007
2. Sep 17, 2007

### dynamicsolo

What is cos (2 pi)? (You're gonna kick yerself...)

P.S. One of the alphas wasn't displaying because you wrote "alpa"; the cos t didn't display because you left out a space between "cos" and "t"; TeX is unforgiving that way...

3. Sep 17, 2007