# Finding the period of 2 multiplied trig functions

## Main Question or Discussion Point

I am trying the period of 2 cosine functions that are multiplied with each other, but I am blanking out on how to find them.

For example, given a function like:

x(t) = cos(10*pi*t)cos(20*pi*t)

I know it has something to do with the frequency of both functions (10*pi & 20*pi), but I cannot remember it and I cannot find it anywhere else. If someone can give me the first part or two of trying to find it, I am hoping it will click.

Any help will be greatly appreciated!

Nick

berkeman
Mentor
I am trying the period of 2 cosine functions that are multiplied with each other, but I am blanking out on how to find them.

For example, given a function like:

x(t) = cos(10*pi*t)cos(20*pi*t)

I know it has something to do with the frequency of both functions (10*pi & 20*pi), but I cannot remember it and I cannot find it anywhere else. If someone can give me the first part or two of trying to find it, I am hoping it will click.

Any help will be greatly appreciated!

Nick

http://www.themathpage.com/atrig/trigonometric-identities.htm

(BTW, I found it using a Google search on: cos a cos b)

Stephen Tashi