posted by John .
for all values of x and y for which both sides are defined.
using the half-angle formulas,
sin(x)-sin(y) = 2 cos((x+y)/2)sin((x-y)/2)
cos(x)-cos(y) = -2sin((x+y)/2)sin((x-y)/2)
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 half-angle for tangent, but got bogged down. Then I saw the sums on the right and went the easy way.