sin21cos9 - cos84cos6 = 1/4 Share with your friends Share 3 Lovina Kansal answered this Dear student Consider, sin 21°cos 9°=12[sin(21°+9°)+sin(21°-9°)] ∵2sin A cos B=sin(A+B)+sin(A-B)⇒sin 21°cos 9°=12sin 30°+sin 12° ........1and cos 84°cos 6°=sin(90°-84°)cos 6°⇒cos 84°cos 6°=sin6°cos 6° ...........2Therefore, sin 21°cos 9°-cos 84°cos 6°=12sin 30°+sin 12°-sin 6°cos 6° from 1 and 2⇒sin 21°cos 9°-cos 84°cos 6°=1212+sin12°-12sin12° sin 2A=2sin Acos A⇒sin 21°cos 9°-cos 84°cos 6°=14 + 12sin12°-12sin12° =14 Regards 3 View Full Answer