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If (1+x+x^2)^n = Sigma(2n)^(r=0) a (r)x^...

If `(1+x+x^2)^n = Sigma_(2n)^(r=0) a _(r)x^r` then prove that
(a) `a_(r)= 20_(2n-r)`
(b) `Sigma_(2n)^(r=0) a_(r) = 1/2 (3^n-a_(n))`

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