# Math Help - Trig inequality.

1. ## Trig inequality.

I'm trying to show that

5+8cos\theta+4cos2\theta+cos3\theta\geq0

but keep going around in circles.

Can anybody take a stab at it?

2. Uhm, what don't you know? Also, what have you tried and didn't work?

Try to use $cos\, 3\theta=4cos^3\theta-3cos\theta$ and $cos\, 2\theta=2cos^2\theta-1$.

3. Originally Posted by Cairo
I'm trying to show that

5+8cos\theta+4cos2\theta+cos3\theta\geq0

but keep going around in circles.

Can anybody take a stab at it?
A start would probably be to express $\cos(2 \theta)$ and $\cos(3 \theta)$ in terms of $\cos(\theta)$.