# Math Help - trig identity help.

1. ## trig identity help.

How do you prove ...

sin(x+y)sin(x-y) = cos ^2y - cos ^2x

1 + cos x = cot x / 2
sin x

tan(x+y)tan(x-y) = sin^2x - sin^2y / cos^2x -sin^2y

(if anyone can tell me the math BBCode that would be nice too)

2. Originally Posted by finalfantasy
How do you prove ...

sin(x+y)sin(x-y) = cos ^2y - cos ^2x
Do you know the formula,
$\sin A \sin B$ now let $A=x+y$ and $B=x-y$.

3. Originally Posted by finalfantasy
1 + cos x = cot x / 2
sin x
Hint: Multiply the top and bottom of the LHS by $1 - cos(x)$.

-Dan