If a(r) is the coefficient of x^(r ) in ...

If `a_(r)` is the coefficient of `x^(r )` in the expansion of `(1+x+x^(2))^(n)(n in N)`. Then the value of `(a_(1)+4a_(4)+7a_(7)+10a_(10)+……..)` is equal to :

`3^(n-1)`

`2^(n)`

`(1)/(3)*2^(n)`

`n*3^(n-1)`

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