Question

(1+x)(1+x^2)(1+x^3)(1+x^4)... find coeff of x^9

Answer 1

(1+x)(1+x2)………(1+x9)

Coefficient will be (1,8)+(1,2,6)+(1,3,5)+(2,7)+(2,3,4)+(3,6)+(4,5)+(0,9)

So coefficient is 8

PS:- (1,8) means forming x9 using x1 and x8 and similarly for others.