# Math Help - Small trigonometry question

1. ## Small trigonometry question

How would I go about solving the equation

cos x = cos 3x ?

I know the equation for cos 2x but this doesn't help - I get $2cos^2(3x/2) - 1$ which doesn't really help. Is there any other way which it can be done?

2. Originally Posted by db5vry
How would I go about solving the equation

cos x = cos 3x ?

I know the equation for cos 2x but this doesn't help - I get $2cos^2(3x/2) - 1$ which doesn't really help. Is there any other way which it can be done?
Sure db5vry,

express Cos(3x) as Cos(2x+x)

expand this using Cos(A+B) and expand Cos(2x) later using Cos(x+x).

3. or turn $\cos x-\cos3x$ by using sum - product formulae.