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If A=18^o, then the value of $\cos^4 A -\sin^4 A$
Option: 1
$\frac{1}{4}\sqrt{10+2\sqrt5}$

Option: 2
$\frac{1}{4}(\sqrt{5}+1)$

Option: 3
$\frac{1}{4}(\sqrt{5}-1)$

Option: 4
$\frac{1}{4}\sqrt{10-2\sqrt5}$

Answers (1)

best_answer

$\\\cos^4 A -\sin^4 A=(\cos^2 A -\sin^2 A)(\cos^2 A + \sin^2 A)\\ =\cos 2A\\ =\cos 36^{\circ}\\ =\frac{\sqrt{5}+1}{4}$

Posted by

SANGALDEEP SINGH

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