(x^2+y)(-y+x^2)-(-x)^2·(-x^2)
=x^4-y^4+x^2*x^2
=2x^4-y^4
(2)(5x-3)(5x+3)-3x(3x-7)
=25x²-9-9x²+21x
=16x²+21x-9
如果本题有什么不明白可以追问,