Given that ,

(a) Find the exact value of

(b) solve the equation for 0< x < 180

Attempted solution

(a) 8cos^310 - 6cos10 = 2(cos3(10))

= 2cos30

=

(b) since 8cos^3x - 6cosx = 2cos 3x

2cos3x = -1

I cosine inversed but am not clear on what to do after that.

I know about the positive and negative quadrants.

However, the 3x is giving me some problems on adding and minus...

Note that 0< x <180