Question

cos3x-3cosx=cos2x+1 {Domain: 0<=x<=360} The solution is 90deg, 180deg, and 270deg, but what are the simplifying steps in between? I know all trig identities.

Answer 1

Explanation:

cos(3x) − 3 cos x = cos(2x) + 1

Use triple and double angle formulas.

(4 cos³x − 3 cos x) − 3 cos x = (2 cos²x − 1) + 1

Simplify.

4 cos³x − 6 cos x = 2 cos²x

4 cos³x − 2 cos²x − 6 cos x = 0

2 cos³x − cos²x − 3 cos x = 0

Factor.

cos x (2 cos²x − cos x − 3) = 0

cos x (cos x + 1) (2 cos x − 3) = 0

Solve.

cos x = 0 → x = 90°, 270°

cos x + 1 = 0 → x = 180°

2 cos x − 3 = 0 → no solution