Question

Rewrite the product as a sum or difference.

20cos(36t) cos(6t)

Answer 1

The applicable identity is ...

... cos(a)cos(b) = (1/2)(cos(a+b) +cos(a-b))

Let a=36t, b=6t and substitute this into your expression. Then you have ...

... 20cos(36t)cos(6t) = 20·(1/2)(cos(36t+6t) + cos(36t-6t))

... = 10(cos(42t) + cos(30t))

... = 10cos(42t) +10cos(30t)