Questions
If the coefficients of three consecutive terms in the expansion of (1+x)n are in the ratio 1:7:42 then the value of n is
a
60
b
70
c
55
d
None of these
detailed solution
Correct option is C
Let (r + 1)th, (r+2)th and (r + 3)th be three consecutiveterms.Then nCr:nCr+1:nCr+2=1:7:42Now nCr nCr+1=17⇒r+1n−r=17⇒n−8r=7 ........(i)And nCr+1 nCr+2=742⇒r+2n−r−1=16⇒n−7r=13 ......(ii)On solving Eqs. (i) and (ii), we get n =55Talk to our academic expert!
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