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प्रश्न
Without using trigonometric tables, find the value of (sin 72° + cos 18°)(sin 72° - cos 18°).
उत्तर
(sin 72° + cos 18°)(sin 72° - cos 18°)
= [ sin (90° - 18°) + cos 18° ][ sin(90° - 18°) - cos 18° ]
= [ cos 18° + cos 18°][cos 18° - cos 18°]
= 0
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