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Trigonometric Ratios

Evaluate: c...

Question

Evaluate: ${cos}^{2} 72^{\circ} - {sin}^{2} 54^{\circ}$

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Solution

cos²72 = cos²(90 - 18) = sin²18
sin²54 = sin²(90 - 36) = cos²36
Firstly find out the value of sin(18°) using the compound angle formulae as shown.

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