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Question

If (1 – x + x2)n = a0 + a1 x + a2 x2 +...+a2n x2n, then a0 + a2 + a4 +...+ a2n equals

(a) 3n+12

(b) 3n-12

(c) 1-3n2

(d) 3n+12

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Solution

Given:
(1 x + x2)n = a0 + a1 x + a2 x2 + ....+a2n x2n ...(1)In order to find a0 + a2 + a4 +....+ a2n
i.e. all even terms are involved
∴ replace x by 1 in equation (1)
we get
(1-1+1)n=a0+a1+a2+ .... +a2ni.e. 1=a0+a1+a2+ .... +a2n ....(2)
and now replace x by –1 in equation (1), we get
(1+1+1)n=a0-a1+a2-a3+ .... +a2ni.e. 3n=a0-a1+a2-a3+ .... +a2n ...(3)

By adding (2) and (3), we get

3n+1=2a0+2a2+2a4+....+2a2ni.e. a0+a2+a4+....+a2n=1+3n2

Hence, the correct answer is option A.


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