Given the integers r>1, n>2,

Question:

Given the integers $r>1, n>2$, and coefficients of $(3 r)^{\text {th }}$ and $(r+2)^{\text {nd }}$ terms in the binomial expansion of $(1+x)^{2 n}$ are equal, then

(A) $n=2 r$

(B) $n=3 r$

(C) $n=2 r+1$

(D) none of these

 

Solution:

(A) $n=2 r$

Explanation:

Given $(1+x)^{2 n}$

$T_{3 r}=T_{(3 r-1)+1}={ }^{2 n} \mathrm{C}_{3 r-1} x^{3 r-1}$

$T_{r+2}=T_{(r+1)+1}={ }^{2 n} C_{r+1} x^{r+1}$

${ }^{2 n} C_{3 r-1}={ }^{2 n} C_{r+1}$

$3 r-1+r+1=2 n$

$n=2 r$

Hence option A is the correct answer.

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