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)
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Originally Posted by finalfantasy How do you prove ... sin(x+y)sin(x-y) = cos ^2y - cos ^2x Do you know the formula, $\displaystyle \sin A \sin B$ now let $\displaystyle A=x+y$ and $\displaystyle B=x-y$.
Originally Posted by finalfantasy 1 + cos x = cot x / 2 sin x Hint: Multiply the top and bottom of the LHS by $\displaystyle 1 - cos(x)$. -Dan
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