第1个回答 2013-02-13
sin6sin42sin66sin78
=sin6sin42cos24cos12
=sin6sin42cos24cos12
=(sin42cos12)(sin6cos24) 按括弧两两 积化和差
=1/2(sin54+sin30)1/2(sin30-sin18)
=1/4(sin54+1/2)(1/2-sin18)
=1/4(1/2sin54+1/4-sin54sin18-1/2sin18)
=1/4[1/2(sin54-sin18)-sin54sin18+1/4] 对小括弧的sin54-sin18再次积化和差
=1/4(cos36sin18-sin54sin18+1/4) cos36=sin54
=1/4*1/4
=1/16本回答被网友采纳
第3个回答 2013-02-11
(1)
解:sin6°sin42°sin66°sin78°
=sin6°cos12°cos24°cos48°
=2cos6°sin6°cos12°cos24°cos48°/2cos6°
=sin12°cos12°cos24°cos48°/2cos6°
=sin24°cos24°cos48°/4cos6°
=sin48°cos48°/8cos6°
=sin96°/16cos6°
=1/16
(2)
解:sin50°(1+√3 tan10°)-cos20°/cos80°√(1-cos20°)
=[sin50°(cos10°+√3 sin10°)/cos10°-cos20°]/√2sin²10°
=(1-cos20°)/√2sin²10°
=√2
第4个回答 2013-02-13
sin2α=2*sinα*cosα cos2α=2*cos^2 α -1 =cos^2 α - sin^2 α= 1-2*sin^2 α tan2α=2*tanα/1-tan^2 α