(cosx+cosy+cosz)/cos(x+y+z)=(sinx+siny+sinz)/sin(x+y+z)=a,则cos(x+y)+cos(y+z)+cos(z+x)的值是用虚数方法做...

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(cosx+cosy+cosz)/cos(x+y+z)=(sinx+siny+sinz)/sin(x+y+z)=a,则cos(x+y)+cos(y+z)+cos(z+x)的值是
用虚数方法做...

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什么叫用虚数方法做?

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