# Homework Help: Proving this trignometric identity

1. Jan 3, 2013

### hms.tech

1. The problem statement, all variables and given/known data

Prove that :

$\frac{cos(x)-1}{(1-cos(x))^{3}}$ = -$\frac{1}{4sin^{4}(0.5x)}$

2. Relevant equations

None that I can think of.
Maybe the double angle formula...

3. The attempt at a solution

I couldn't do much in this question :

-$\frac{1}{(1-cos(x))^{2}}$

2. Jan 3, 2013

### micromass

Things to try: expand $(1-\cos(x))^2$ and expressing $\sin(x/2)$ in terms of just sin and cos.

3. Jan 3, 2013

### ehild

Yes, use the double angle formula.

ehild

4. Jan 5, 2013

### hms.tech

What exactly do you mean by that ?

Should I expand it in powers of Sin(x)/Cos(x) ?

5. Jan 5, 2013

### Saitama

Use double angle formula. What is cos(2x) in terms of sin? Similarly, what is cos(x)?