Given:
sin21∘cos9∘−cos84∘cos6∘ = ?
Formula used:
sinxcosy=21[sin(x+y)+sin(x−y)]
cosxcosy=21[cos(x+y)+cos(x−y)]
Calculation:
sin21∘cos9∘=21[sin(21∘+9∘)+sin(21∘−9∘)]
cos84∘cos6∘=21[cos(84∘+6∘)+cos(84∘−6∘)]
⇒ sin21∘cos9∘=21[sin30∘+sin12∘]
⇒ cos84∘cos6∘=21[cos90∘+cos78∘]
⇒ sin21∘cos9∘−cos84∘cos6∘=21[sin30∘+sin12∘]−21[cos90∘+cos78∘]
⇒ 21[21+sin12∘]−21[0+cos78∘]
⇒ 21[21+sin12∘]−21cos78∘
⇒ 21[21+sin12∘−cos78∘]
⇒ 21[21+sin12∘−sin12∘]
⇒ 21×21=41
∴ The correct answer is option (2).