1.

In the binomial expansion of `(1+x)^n`, coefficients of the fifth, sixth and seventh terms are in A.P. find all the values of `n`for which this can happen.

Answer» Coefficients of fifth, sixth and seventh terms in `(1+x)^n` are `C(n,4),C(n,5) and C(n,6).`
As, they are in `A.P.`,
`:. 2**C(n,5) = C(n,4)+C(n,6)`
`=>2*(n!)/(5!(n-5!)) = (n!)/(4!(n-4!)) + (n!)/(6!(n-6!))`
`=>2*1/(5(n-5)) = 1/((n-4)(n-5)) + 1/(6*5)`
`=>2/(5(n-5)) = 1/((n-4)(n-5)) + 1/30`
`=>2/(5(n-5)) = (30+(n-4)(n-5))/(30(n-4)(n-5))`
`=>12(n-4) = 30+(n^2+20-9n)`
`=>12n-48 = n^2+50 -9n`
`=>n^2-21n+98 =0`
`=>n^2-14n-7n+98 =0`
`=>(n-14)(n-7) = 0`
`=> n = 14 or n = 7`
So, `n` can be `14` and `7` for the given expansion.


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