F4,,A-MATHS,,TRIGO!!,,急問喔

cot(x-y)=kcotx
cos(x-y)/sin(x-y)=kcosx/sinx
so cos(x-y)=kcosx--(1)
and sin(x-y)=sinx----(2)

LHS
=sinx cos(x-y)
=sin(x-y) kcosx<---put formula(1)and(2)into the originality formula..
=ksin(x-y) cosx
=RHS

so sinx cos(x-y)=ksin(x-y)cosx is ture

cot(x-y)=kcotx
1/tan(x-y)=k/tanx
tanx=ktan(x-y) (By cross multipication)
sinx/cosx=ksin(x-y)/cos(x-y)
sinxcos(x-y)=ksin(x-y)cosx (By cross multipication again)
In which the identity is proven