1.

If a_k is the coefficient of x^k in the expansion of (1 +x+x^2)^n for k = 0,1,2,……….,2n then a_1 +2a_2 + 3a_3 + ………+2n.a_(2n) =

Answer»

`-a_0`
`3^n`
`n.3^n`
`-n.3^n`

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