Question

Prove that Cos (x + y ) = COS X COS y - sin x sin y​

Answer 1

Answer:

cosx cosy =1/2 ( cos( x+y) + cos( x-y)

sinx siny = 1/2 ( cos(x-y) - cos( x+y)

=>cosx cosy - sinx siny = 1/2 ( cos( x+y)+ cos(x-y) -cos(x-y) + cos(x+y)

=>1/2 ( 2 cos(x+y)

=> cos(x+y) Proved