(adsbygoogle = window.adsbygoogle || []).push({}); When you multiply two cos waves together, where do all the "other" frequency's come f

When you multiply two cos waves with similar frequency's you get a graph that looks like this:

http://www.wolframalpha.com/input/?i=(cos(2*pi*0.8*x))*(cos(2*pi*0.9*x))

With equation y=(cos(2*pi*0.8*x))*(cos(2*pi*0.9*x))

Now if you analyse this wave with a Fourier transform to see the the frequency domain there are lots of other frequency's as well as the 0.8HZ and the 0.9HZ that seem to have come from no where.

Can someone please show me the maths that shows where these frequency's come from?

Thanks

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# When you multiply two cos waves together, where do all the other frequency's come f

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