用提公因式法.x(x+x)(x-y)-x(x+y)^2

解
x(x+x)(x-y)-x(x+y)^2
=x(x+y)[(x-y)-(x+y)]
=x(x+y)(-2y)
=-2xy(x+y
a(x-a)+b(a-x)-c(x-a)
=a(x-a)+b(a-x)+c(a-x)
=(a-x)(a+b+c)
5×3^4+36×3^2+9×3^2
=45×3²+36×3²+9×3²
=(45+36+9)×3²
=90×9
=810
39×37-13×81
=13×3×37-13×3×27
=13×3×(37-27)
=13×3×10
=390