4 cos^2 x - 3 = 0 on the interval (90 degrees, 180 degrees) ------> Find all solutions

Answer 1

4 cos²x - 3 = 0

4 cos²x = 3

cos²x = ³/₄

cos x = ±^√3/₂

From the unit circle, the only place on the interval [90°, 180°] where cos x = ± ^√3/₂ is where x = 150° (or ^5π/₆ in radians)