第31页 - 启东中学作业本七年级数学人教版

解：$5x^2y-2xy^2+(-2xy^2+4x^2y)=5x^2y-2xy^2-2xy^2+4x^2y=9x^2y-4xy^2$

解：$(3x^2+x-5)-(4-x+7x^2)=3x^2+x-5-4+x-7x^2=-4x^2+2x-9$

解：$(2x^2+xy+3y^2)-(x^2-xy)=2x^2+xy+3y^2-x^2+xy=x^2+2xy+3y^2$

解：$(2a^2-4ab+b^2)-(-3a^2+2ab-5b^2)$

$=2a^2-4ab+b^2+3a^2-2ab+5b^2$

$=5a^2-6ab+6b^2$

解：$M=(-x^2+3xy-\frac 12y)-(-\frac 12x^2-xy+y)$

$=-x^2+3xy-\frac 12y+\frac 12x^2+xy-y$

$=-\frac 12x^2+4xy-\frac 32y$

解：$M-2N=(3x^2+2x-1)-2(-x^2-2+3x)$

$=3x^2+2x-1+2x^2+4-6x$

$=5x^2-4x+3$

解：$3A-B=3(4x^2-5xy+5y^2)-(x^2-2xy+7y^2)$

$=12x^2-15xy+15y^2-x^2+2xy-7y^2$

$=11x^2-13xy+8y^2$

解：$2A-3B=2(x^2+xy+y^2)-3(-3xy-x^2)$

$=2x^2+2xy+2y^2+9xy+3x^2$

$=5x^2+11xy+2y^2$