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Question: Answered & Verified by Expert
The smallest natural number n , such that the coefficient of x in the expansion of x2+1x3n is C23 n , is
MathematicsBinomial TheoremJEE MainJEE Main 2019 (10 Apr Shift 2)
Options:
  • A 58
  • B 38
  • C 35
  • D 23
Solution:
1935 Upvotes Verified Answer
The correct answer is: 38

In the expansion of x2+1x3n the general term is Tr+1=Cr nx2n-r1x3r

=Cr nx2n-2r-3r=Cr nx2n-5r

For coefficient of x, 2n-5r=1

r=2n-15

So, we have the coefficient as C2n-15 n

Using, the given value and Crn=Cn-rn

C2n-15 n=C23=Cn-23 n n

If 2n-15=23n=58 and if 2n-15=n-23n=38

Thus, the minimum value of 'n' is 38.

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