Solve the equation cos2x - 5cosx = 2
cos2x= 2cos^2x - 1
2cos^2x - 1 - 5cosx = 2
Let cosx = y
2y^2 - 5y - 3 = 0
(2y + 1)(y + 3) = 0
y = -1/2 or -3
cosx = -1/2 or -3
x = 120 degrees
PV
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