show that the given equation is not an identity Hint for eac

show that the given equation is not an identity. (Hint: for each equation, find values of s and t for each equation is not true.) Cos (s+t) =Cos s + Cos t

Solution

cos(s+t)

Let s=30degree, t=60degree

Therefore cos(30+60)=cos 90=0

And cos 30+cos 60 =(sqrt3)/2 +(1/2)=(sqrt 3 +1)/2

Therefore cos(30+60) not equals to cos 30 +cos 60

Hence the given is not an identity.