Thread: Solving trigonometric equations

Hi,

I have the solve the following equation without using a calculator:

2cos2x=1+cosx, where x is in radians and between 0 and 2 pi.

I used the double angle formulae to get that:

(4cosx +3)(cosx -1)=0,

But how can I figure out what cosx = -3/4 is?

Con somebody help me? Is there an easier way of doing it? Any help is very much appreciated.

Alas, you won't be able to find that by hand, there's where the calculator comes handy.

There are certain angles easily found, but this is not the case.