Question

Solve cos2θ + cosθ = 0 for 0° ≤ θ < 90°.

Answer 1

Use Cos 2 theta = 2 Cos^2 theta -1 to get a quadratic in theta.

Answer 2

COS2A = 2COSA^2 - 1

PUT THAT INTO THE EQUATION

2COSA^2 +COSA -1 =0

PUT COSA = X
2X^2 +X -1 =0
2X(X +1) -(X+1)=0
X= -1 , I/2

COSA = -1 , 1/2

A= pi, pi/3

Answer 3

COSA^2 IS FOR (COS^2A)