(a-2)(a²+a+1)+(a²-1)(2-a)
=[(a-1)-1](a²+a+1)+(a²-1)(2-a)
=(a-1)(a²+a+1)+(a²-1)(2-a)-(a²+a+1)
=a³-1-a³+2a²+a-2-(a²+a+1)
=2a²+a-3-(a²+a+1)
=a²-4
=(a+2)(a-2)
=20*16
=320
(3x²y³)*(-4x^m y^4)*(5xy^n)=-60x^5y^10
3*(-4)*5*x^(3+m)y^(7+n)=-60x^5y^10
3+m=5
7+n=10
解得 m=2,n=3对不起，一共有两问，对不起，第一问a=18