$c o s^{4} A - s i n^{4} A$ is equal to

s i n 2 A

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- s i n 2 A

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c o s 2 A

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- c o s 2 A

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Solution

The correct option is C $c o s 2 A$
${cos}^{4} A - {sin}^{4} A = {({cos}^{2} A)}^{2} - {({sin}^{2} A)}^{2} = ({cos}^{2} A + {sin}^{2} A) ({cos}^{2} A - {sin}^{2} A) = {cos}^{2} A - {sin}^{2} A = cos 2 A$

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