问题描述: 证明:sinπ/9[(sinπ/9)+sin(3π/9)]=sin2(2π/9)等号后边是sin平方(2π/9) 1个回答 分类:数学 2014-09-30 问题解答: 我来补答 此题用到和差化积公式和倍角公式和差化积公式:sin(a+b)+sin(a-b)=2sina*cosb,也可写成sina+sinb=22sin(a+b)/2*cos(a-b)/2倍角公sin(2a)=2sina*cosa套用公式即可解题如下:sinπ/9[(sinπ/9)+sin(3π/9)]=sin(π/9)* 2sin [(π/9+3π/9)/2] cos[(π/9-3π/9)/2= sin(π/9)*2sin (2π/9)cos(π/9) =2sin(π/9)cos(π/9) sin (2π/9)=sin (2π/9)*sin (2π/9)=sin 2 (2π/9) 展开全文阅读