Question

prove the equation equel to zero

$\cos(36) \times \cos(54) - \sin(36) \times \sin(54)$

Answer 1

Hi,

Here is your answer !
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cos36° × cos54° - sin36° × sin54°

= sin(90°-36°) × sin(90°-54°) - sin36° × sin54°

= sin54° × sin36° - sin54° × sin36°

= 0
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IDENTITY USED :

cosA = sin(90° - A)