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Question
Without using trigonometric tables, prove that:
cos 81° – sin 9° = 0
Theorem
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Solution
LHS = cos 81° − sin 9° = 0
= `cos (90^circ - 9^circ) - sin 9^circ`
= `sin 9^circ - sin 9^circ`
= 0
= RHS
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