Ok, I have a few problems I just cant seem to understand.

1. Find the Exact Value of the Expression

Cos(tan^-1(4/3))

I know your supposed to write tangent in terms of cosine but im completely lost.

2. Find all exact solutions (in simplest form) to the following equations.

a) 2cosxsinx +cosx = 0

b)sin2x = tanx

I tried on each, and failed miserably on each. Any help is appericiated.