Precalculus
posted by John .
Show that
cot((x+y)/2)=(sin(x)sin(y))/(cos(x)cos(y))
for all values of x and y for which both sides are defined.

using the halfangle formulas,
sin(x)sin(y) = 2 cos((x+y)/2)sin((xy)/2)
cos(x)cos(y) = 2sin((x+y)/2)sin((xy)/2)
tada! 
Wow, I tried that but I messed up signs! Thanks!

Oops I meant sum and difference

No, I meant sum and difference. I started out trying the halfangle for tangent, but got bogged down. Then I saw the sums on the right and went the easy way.
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