Question 609772
Use the identity cos^2(x) - sin^2(x) = cos(2x) to get



cos^2(x) - sin^2(x) = cos(2x)


cos^2(40) - sin^2(40) = cos(2*40)


cos^2(40) - sin^2(40) = <font color="red">cos(80)</font>

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