How to prove this trig equation

[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 don't 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

 

[Integral]

From the information you have provided it seems like all you need to do is evaluate the sin and cos at [itex] \frac \pi {10} [/itex]

 

[mohlam12]

but i can't use the calculator... because the second question doesn't say CALCULATE, in fact, it says NOTE from the previous question what sin and cos pi/10 is!

 

[ivybond]

x = [itex]\frac \pi {10} [/itex]
[itex]3x = \frac \pi {2} - 2x [/itex]