解由题知展开式4项故(1+ax)的次数为3故由(1+ax)^3=1+10x+bx^2+a^3x^3由10x的系数为10.又由C(3,1)1^2(ax)^1=3ax故3a=10即a=10/3又由C(3,2)1^1(ax)^2=3×a^2x^2=3×(10/3)^2x^2故bx^2的系数b=3×(10/3)^2=100/3.
