Question

intresting binomial question
find the coefficient of x^3 in (1+x+x^2)^7

Answer 1

i did this
(1+x+x^2)^7 = [(1-x^3)^(7)]*[(1-x)^(-7)]
now i dont know what to do for negetive index i.e x^-7 pl help

Answer 2

Suggestion: Try calculating the first two or three powers of that trinomial and see if you can see a pattern in the coefficients of the result.