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# Trigonometry problem watch

1. The answer is cos2x. Why?
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2. (Original post by superkillball)
cos^4 - sin^4 = (cos^2 - sin^2)(cos^2 + sin^2)

cos^2 + sin^2 = 1, so

cos^4 - sin^4 = cos^2 - sin^2 = cos2x
3. Maybe something to do with difference of two squares?
I'm not sure.
What module is it?
4. Ah well done James, I shouldn have seen that.
5. (Original post by JamesF)
cos^4 - sin^4 = (cos^2 - sin^2)(cos^2 + sin^2)

cos^2 + sin^2 = 1, so

cos^4 - sin^4 = cos^2 - sin^2 = cos2x
thanks a lot

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