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.

Dear HighlyFlammable,

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.

3. I did that and got 4cos^2x - 5cosx - 5 = 0 and I'm now unsure how to factorise it. Am I missing something obvious? I'm having a bit of a slow day.

4. Originally Posted by HighlyFlammable
I did that and got 4cos^2x - 5cosx - 5 = 0 and I'm now unsure how to factorise it. Am I missing something obvious? I'm having a bit of a slow day.
Incorrect. It should be, $4\cos^{2}x-5\cos x-6=0$