QT P1 M 4

the expression=(1+x)(1+2x²+x⁴)(1+3x³+3x^{6}+x^{9})(1+4x^{4}.....)(1+5x^{5}....)(1+6x^{6}.....)
(As frm n=7 onwards, der will be no x^7 term, frm d last 3 expresions, I have negleted powers greater than 6)

on comparing co-efficients,1*6x^{6}=6

x*5x^{5}=5

2x²*4x⁴=8
3x³*3x³=9
so, coeffic of x^{6}=6+5+8+9=28

but in d ques, it should hav been mentioned that n→∞, or atleast n≥6

ya atleast in jee we wont encounter such probs!!!