# trig identity help.

• Nov 5th 2007, 01:52 PM
finalfantasy
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)
• Nov 5th 2007, 04:38 PM
ThePerfectHacker
Quote:

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 $B=x-y$.
• Nov 6th 2007, 03:42 AM
topsquark
Quote:

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