Trigonometry
posted by Katlynn .
Use inverse trigonometric functions to find the solutions of the equation that are in the given interval, and approximate the solutions to four decimal places. (Enter your answers as a commaseparated list.)
cos(x)(9cos(x) + 4) = 4; [0, 2π)

expand first
9cos^2 x + 4cosx  4 = 0
let y = cosx
9y^2 + 4y  4 = 0
y = (4 ±√160)/18 , using the formula
= (4 + 4√10)/18
= (2 ± 2√10)/9
= appr .480506 or appr .92495
then cosx = .480506 or cosx = .92495
x = 61.28° or 298.72° or 157.66° or 202.34°