# Math Help - Trigonometric Equation

1. ## Trigonometric Equation

Hi there,

I'm stuck on this question:

Solve 2cos2x - 5cosx - 4 = 0 for 0≤ x < 2π.

2. Originally Posted by HighlyFlammable
Hi there,

I'm stuck on this question:

Solve 2cos2x - 5cosx - 4 = 0 for 0≤ x < 2π.

Expand cos2x using, $\cos 2x=2\cos^{2}x-1$ and you will get a quadratic equation of cos x. Solve it and you can find a value for x.
Incorrect. It should be, $4\cos^{2}x-5\cos x-6=0$