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If the coefficient of 4th term in the ex...

If the coefficient of 4th term in the expansion of `(a+b)^n` is 56, then n is

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The correct Answer is:
`n = 8`
`T_(4) = T_(3+1) = .^(n)C_(3)a^(n-3)b^(3)`
`rArr .^(n)C_(3) = 56 rArr (n!)/(3!(n-3)!) = 56`
or `n(n-1) (n-2) = 56 xx 6`
or `n(n-1)(n-2)=8xx7xx6`
or `n = 8`
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