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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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