Question

${sen}^{2} a - {cos}^{2} a$
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Answer 1

$\sin {}^{2} (a) - \cos {}^{2} (a) = \sin(a) {}^{2} - \cos(a) {}^{2}$

$= - ( \cos(a) {}^{2} - \sin(a ) {}^{2} )$

$= - \cos(2a)$