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Matematika XI
Gunakan rumus sebagai berikut:
sin A - sin B = 2.cos[(A+B)/2].sin[(A-B)/2]
cos A + cos B = 2.cos[(A+B)/2].cos[(A-B)/2]
= [sin 120° - sin 30°] / [cos 225° + cos 15°]
= { 2.cos[(120°+30°)/2].sin[(120°-30°)/2] } / { 2.cos[(225°+15°)/2].cos[(225°-15°)/2] }
= [2.cos 75°.sin 45°] / [2.cos 120°.105°]
= [cos 75°.sin 45°] / [cos 120°.cos(180°-75°)]
= [cos 75°.sin 45°] / [cos 120°.(-cos 75°)]
= - sin 45° / cos 120°
= - [√2/2] / [- 1/2]
= √2