y（y+1）（x²+1）+x（2y²+2y+1）
=y(y+1)x²+(2y²+2y+1)x+y(y+1)
=(yx+y+1)[(y+1)x+y]
=(xy+y+1)(xy+x+y)能问一下是怎么十字相乘的吗把x²的系数y(y+1)分解为y,(y+1)
y (y+1)
(y+1) y
y•y+(y+1)•(y+1)=(2y²+2y+1)(x项的系数）
∴原式=[yx+(y+1)]•[(y+1)x+y]