Question
Without using trigonometric tables, find the value of (sin 72° + cos 18°)(sin 72° - cos 18°).
✓
Answer
(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
= ( sin (90° - 18°) + cos 18° )( sin(90° - 18°) - cos 18° )
= ( cos 18° + cos 18°)(cos 18° - cos 18°)
= 0
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