证明cosx-cos(x+2y)/2siny= sin(x+y)证明cosX-cos(x+2y)/2siny= sin(x+y)
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证明cosx-cos(x+2y)/2siny=sin(x+y)证明cosX-cos(x+2y)/2siny=sin(x+y)证明cosx-cos(x+2y)/2siny=sin(x+y)证明cosX-
证明cosx-cos(x+2y)/2siny= sin(x+y)证明cosX-cos(x+2y)/2siny= sin(x+y)
证明cosx-cos(x+2y)/2siny= sin(x+y)
证明cosX-cos(x+2y)/2siny= sin(x+y)
证明cosx-cos(x+2y)/2siny= sin(x+y)证明cosX-cos(x+2y)/2siny= sin(x+y)
cosx=cos[(x+y)-y]=cos(x+y)cosy+sin(x+y)siny
cos(x+2y)=cos[(x+y)+y]=cos(x+y)cosy-sin(x+y)siny
cosx-cos(x+2y)/2siny
=[cos(x+y)cosy+sin(x+y)siny-(cos(x+y)cosy-sin(x+y)siny)]/2siny
=2sin(x+y)siny/2siny
=sin(x+y)
=右边
所以原等式成立
分母的化简
cosx－（cosx cos2y- sinx sin2y）
然后用二倍角公式
cos2y= 1－2 sin^2y
sin2y= 2siny cosy
代入分母展开既可
除以分子，然后合并就完了
证明cosx-cos(x+2y)/2siny= sin(x+y)证明cosX-cos(x+2y)/2siny= sin(x+y)
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