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 nCr−1=28,nCr=56 and nCr+1=70 ∴nCrnCr−1=n−r+1r=5628=2and nCr+1nCr=n−rr+1=7056=54 This gives n+1=3r and 4n−5=9r ∴4n−5n+1=3⇒n=8