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Question

If C0,C1,C2,Cn are coefficients of expansion (1+x)n then prove that:
C0+3.C1+5.C2++(2n+1).Cn=(n+1)2n.

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Solution

L.H.S.
=nC0+3.C1+5.C2+...+(2n+1)nCn

=nC0+(2+1)nC1+(4+1)nC2+...(2n+1)nCn

=nC0+(2.nC1+nC1+(4.nC2+nC2)+...(2n.nCn+nCn)

=2.nC1+4nC2+6nC3+...+2nnCn+(nC0+nC1+nC2+nC3+...nCn)

=2[nC1+2nC2+3.nC3+...+nCn]+(1+1)n

=2[n+2n.(n1)2!+3n.(n1)(n2)3!+...+nterms]+2n

=2n[1+(n1)+(n1)(n2)2!+...+nterms]+2n

=2n.(1+1)n1+2n=2n.2n1+2n+n.2n+2n

=(n+1)2n=R.H.S.

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