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Question

If the three consecutive coefficient in the expansion of (1+x)n are 28,56 and 70, then the value of n is

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Solution

Let the three consecutive coefficient be
nCr1=28, nCr=56 and nCr+1=70
nCrnCr1=nr+1r=5628=2and nCr+1nCr=nrr+1=7056=54
This gives n+1=3r and 4n5=9r
4n5n+1=3n=8

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