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December 18, 2014

December 18, 2014

Posted by **John** on Monday, February 10, 2014 at 8:08pm.

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.

- Precalculus -
**Steve**, Monday, February 10, 2014 at 8:34pmusing 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)

ta-da!

- Precalculus -
**John**, Monday, February 10, 2014 at 9:03pmWow, I tried that but I messed up signs! Thanks!

- Precalculus -
**John**, Monday, February 10, 2014 at 9:09pmOops I meant sum and difference

- Precalculus -
**Steve**, Tuesday, February 11, 2014 at 12:27amNo,

**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.

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