If $(1+x+x^2)^n= \Sigma_{r=0}^{2n}a_r x^r$ then $a_r- \binom n1 a_{r-1}+\binom n2 a_{r-2}-\binom n3 a_{r-3}+....+(-1)^r\binom nr a_{0} $ is equal to (r is not a multiple of 3)
Options
a)0
b) $^nC_r$
c) $a_r$
d)1
There's no way I can think of to get an expression like that. I tried differentiating to see whether it could help me or not but it was in vain. Please help me out of this problem.