这个可根据十字相乘法看出:-10y^2=-2y*5y
而-2+5=3
也可这样:
=x^2+(5xy-2xy)-10y^2
=(x^2+5xy)-(10y^2+2xy)
=x(x+5y)-2y(5y+x)
=(x-2y)(x+5y)