# Math Help - Solving Trig Function Problem "by hand" (given values, no calculator)

1. ## Solving Trig Function Problem "by hand" (given values, no calculator)

Given sin( π/5 ) = 0.588 and cos ( π/8 ) = 0.924

How can you figure out cos ( 27π/8 )?

I can't think of any trig identities/formulas that would fit the problem. Help please?

(π is pi btw)

2. I can't straight away see how this one can be solved.

But I would suggest having a look at the indentities for $\cos(a+b)$ and $\cos(a-b)$

3. ohh I got it now. I never thought about the addition identities! I got the right answer now. Thanks!