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Question

Let (1+x2)2.(1+x)n=n+4k=0ak.xk. If a1,a2 and a3 are in A.P., find n.

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Solution

(1+x2)2(1+x)n
=(1+2x2+x4)(1+nx+nC2x2...)
Hence
a1=nC1=n
a2=2+nC2.
a3=nC3+2nC1
Now
2a2=a1+a3
Or
4+2nC2=3nC1+nC3
4+(n(n1))=3n+n(n1)(n2)6
43n=n(n1)[n261]
43n=n(n1)(n8)6
2418n=n(n29n+8)
n39n2+8n24+18n=0
n39n2+26n24=0.
(n2)(n3)(n4)=0
n=2, n=3, n=4.
Hence, n=2, 3, or 4

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