解;(1)2x2(x2-x+1)-x(2x3-10x2+2x), =2x4-2x3+2x2-(2x4-10x3+2x2), =8x3, 把x=-代入原式得: 原式=8x3=8×(-)3=-1;
(2)xn(xn+9x-12)-3(3xn+1-4xn), =x2n+9xn+1-12xn-9xn+1+12xn, =x2n; 把x=-3,n=2代入得出: 原式=x2n=(-3)2×2=81;
(3)∵3x(xm+5)=3x6+5nx, ∴3xm+1+15x=3x6+5nx, ∴m+1=6,15=5n, 解得:m=5,n=3, 则m+n的值是:5+3=8. |