Question 5

Find the $$cos^{4}A-sin^{4}A$$?

Solution

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 $$cos^{4}A-sin^{4}A$$ = ( $$cos^{2}A-sin^{2}A$$ ) ( $$cos^{2}A+sin^{2}A$$)

We know that, $$cos^{2}A - sin^{2}A$$ = cos 2(A)

and $$cos^{2}A + sin^{2}A$$ =  1

Therefore,
 $$cos^{4}A-sin^{4}A$$ = cos2A

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