- #1

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## Homework Statement

cosx - cosy=-2 sin(x + y/2) sin(x - y/2)

## Homework Equations

dont know what identities to use

## The Attempt at a Solution

ok so when i figure it out, the RHS always comes out to either...

(cos x sin y - sin x cos y)/2

or just

cos x sin y - sin x cos y without the divided by 2 part

it never equals the LHS=cosx - cosy