# Math Help - verifying

1. ## verifying

Can somebody help me verify this?
-2cos^2xtan(-x)=sin2x

2. Originally Posted by unmcintosh
Can somebody help me verify this?
-2cos^2xtan(-x)=sin2x
knowing $\tan(-x)=-tan(x)=\frac{sin(x)}{\cos(x)}$

we multiply by cos²(x) to get

$-\sin(x)\cos(x)$

and then dont forget that negative two

$2\sin(x)\cos(x)=\sin(2x)$

QED

3. when you multiply by cos^2x, how do you get -sinxcosx?

4. Originally Posted by unmcintosh
when you multiply by cos^2x, how do you get -sinxcosx?
$\cos^2(x)\cdot\tan(-x)=\cos^2(x)\cdot\frac{-\sin(x)}{\cos(x)}=-\sin(x)\cos(x)$