If $(1 -x + x^2)^n = a_0 + a_1x + a_2x^2 + ....... + a_{2n}x^{2n}$, then $a_0 + a_2 + a_4 +........+ a_{2n}$ is equal to
If $(1 -x + x^2)^n = a_0 + a_1x + a_2x^2 + ....... + a_{2n}x^{2n}$, then $a_0 + a_2 + a_4 +........+ a_{2n}$ is equal to
- A
$\frac{1}{2} (3^n+1)$
- B
$\frac{1}{2} (3^n-1)$
- C
$\frac{1}{2} (1-3^n)$
- D
$\frac{1}{2} +3^n$
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