4、_________________,第n项为________________.参考答案1. B 2. D 3. B 4.(1)-+x+2x2+x3-5x4(2)-5x4+x3+2x2+x-5.(1)-y4-2xy2-5x2y3+3x3y+x43(2)x4+3x3y-2xy2-5x2y3-y46.-6(a-b)4+2(a-b)3-3(a-b)2+(a-b)-17.解:(1)-x3+3x2-5xy2-y3;(2)当x=-1,y=-2时,原式=3×(-1)2-(-2)3-5×(-1)×(-2)2-(-1)3=32.8.解