Question

求(1-2X)的5次幂乘以(1+3X)的4次幂的展开式中按X的升幂排列的前3项

Answer 1

1+2x-26x^2

1*1=1

[5*(-2)*1+4*3*1]*x=2x

[(5*4/2)*2^2+5*(-2)*4*3+(4*3/2)*3^2]x^2=[40-120+54]x^2=-26x^2