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Question
Without using trigonometric tables, prove that:
cos 54° cos 36° – sin 54° sin 36° = 0
Theorem
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Solution
LHS = cos 54° cos36° - sin 54° sin 36°
= cos (90° - 36°) cos 36° - sin (90° - 360°) sin 36°
= sin 36° cos 36° - cos 36° sin 36°
= 0
= RHS
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Chapter 12: Trigonometric Ratios of Some Complemantary Angles - EXERCISE 12 [Page 590]
