my question is this...

$\displaystyle

\cos x - \cos y=-2 \sin\left (\frac{x + y}{2}\right)\sin\left (\frac{x - y}{2}\right)

$

how do you make LHS equal RHS???

i end up with the RHS equaling

$\displaystyle

\frac{\cos x \sin y - \sin x \cos y}{2}

$

or

$\displaystyle

\cos x \sin y - \sin x \cos y

$ without the divided by 2

please help i need to figure it out by tomorrow...