# Trig identity proof.

1. Aug 18, 2012

Prove:
$\frac{CosθSinθ}{1 + Tanθ}$ = Cos2θ
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I multiply out the denominator to get:

CosθSinθ = Cos2θ + CosθSinθ

I cannot seem to prove it.

Starting to think it's a trick question.. :/

2. Aug 18, 2012

### krome

It most certainly is a trick question because the supposed identity doesn't even work if you plug in theta = 0. The left side is 0 while the right is 1, in this case!

3. Aug 18, 2012

### Millennial

You might be asked to solve the equation instead of proving the identity, and the solutions of the equation are the zeros of cosine, id est odd integer multiples of $\pi/2$.