下列各式由等号左边变到右边变错的有( )①a-(b-c)=a-b-c②(x2+y)-2(x-y2)=x2+y-2x+y2③-(a+b)-(-x+y)=-a+b+x-y④-3(x-y)+(a-b)=-3x-3y+a-bA.1个B.2个C.3个D.4个
根据去括号的法则:
①应为a-(b-c)=a-b+c,错误;
②应为(x2+y)-2(x-y2)=x2+y-2x+2y2,错误;
③应为-(a+b)-(-x+y)=-a-b+x-y,错误;
④-3(x-y)+(a-b)=-3x+3y+a-b,错误.
故选D.
①应为a-(b-c)=a-b+c,错误;
②应为(x2+y)-2(x-y2)=x2+y-2x+2y2,错误;
③应为-(a+b)-(-x+y)=-a-b+x-y,错误;
④-3(x-y)+(a-b)=-3x+3y+a-b,错误.
故选D.
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