# How to prove this trig equation

1. Oct 26, 2005

### mohlam12

hey
i have a problem with composed with two questions:
first i havr to prove that
cos(3x)=cos(x).(4cos^2(x) - 3)
okay, for this one, I was able to prove it; then the second question came, saying:
what is sin(pi/10) and cos(pi/10)
i dont know how to do that, but it should be related to the question before, how!? i don't know!
anyone with something to say ... ??
thanks

2. Oct 26, 2005

### Integral

Staff Emeritus
From the information you have provided it seems like all you need to do is evaluate the sin and cos at $\frac \pi {10}$

3. Oct 26, 2005

### mohlam12

but i cant use the calculator... because the second question doesnt say CALCULATE, in fact, it says NOTE from the previous question what sin and cos pi/10 is!!

4. Oct 26, 2005

### ivybond

x = $\frac \pi {10}$
$3x = \frac \pi {2} - 2x$

Last edited: Oct 26, 2005