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If the coefficients of a^r-1, a^r and a^...

If the coefficients of `a^r-1`, `a^r` and `a^r+1` in the expansion of `(1+a)^n` are in arithmetic progression, prove that `n^2` - n(4r+1)+`4r^2` - 2 =0.

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The correct Answer is:
`n^2 - n (4r + 1) + 4r^2 - 2 = 0`
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