Proving Trigonometric Identities

[mathnewb]

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

 

[yeongil]

I asked in your previous thread https://www.physicsforums.com/showthread.php?t=309555" if you mean (x+y)/2 or x+(y/2), but you didn't answer. I also think that the rest of us here would like to see the steps you took to get to
(cos x sin y - sin x cos y)/2 or cos x sin y - sin x cos y.


01

 

[mathnewb]

i mean ((x+y)/2)