①原式=24+6+(-14-16) =30-30 =0; ②原式=-18+20-21 =-39+20 =-19; ③原式=9-[(-)+(-)]×12 =9-(-8-3) =20; ④原式=0.25×(-8)-[4÷+1]+(-1) =-2-10-1 =-13; ⑤原式=x3y+xy2-(2x3y-4xy2) =x3y+xy2-2x3y+4xy2 =-x3y+5xy2; ⑥原式=3y2-x2+2x-y-x2-3y2 =-2x2+2x-y; ⑦原式=-2x3+4x-x2-x-3x2+2x3 =3x-4x2 当x=-3时, 上式=3×(-3)-4×(-3)2 =-9-36 =-45. |