which expression completes the identity sin ucosv=?

Answer 1

we have
sin(u+v) = sinu cosv + cosu sinv
sin(u-v) = sinu cosv - cosu sinv
add them up and you have

sin(u+v)+sin(u-v) = 2sinu cosv

now just finish it up.

Answer 2

Which expression completes the identity sin u + sin v = ?