A ratio of the 5 t h term from the beginning to the 5 t h term from the end in the binomial expansion of 2 1…

A ratio of the 5th term from the beginning to the 5th term from the end in the binomial expansion of 213+1231310 is
  1. 1:41613
  2. 43613:1
  3. 23613:1
  4. 1:2613

Solution

Total number of terms in the expansion of 213+1231310=10+1=11

Now, 5th term from end =11-5+1=7th from beginning.

We need to find a ratio of the 5th term from the beginning to the 5th term from the end (i.e. 7th term from beginning) in the binomial expansion of 213+1231310=T5T7

=10C421310-412313410C621310-6123136  Tr+1=nCran-rbr

=10!4!6!×22×12434310!6!4!×243×12632

=43613:1

Asked in: JEE Main 2019 (12 Jan Shift 1)

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