ban oi minh lam den day roi nhung ko biet lam nua ban thu nghi tiep xem nhe
Cos 3xcosx^{3} +sin 3x sinx^{3}= :frac{1}{2 :sqrt{2} }
sau do tach cos^3 =cos^2 cosx tuong tu doi voi sinx
va cuoi cung ra la cos4x+ cos2x^{2} =cos( :frac{ }{4} = :frac{1}{ :sqrt{2} }con lai ban tu lam nhe
ko bit co dung ko minh cung dang co gang hoc pan nay co bai nao ban gui cho minh nhe
cam on ban rat nhieu