Question

In the binomial expansion of $(a-b{)}^n,n\ge 5$, the sum of the $5^{th}$ and $6^{th}$ terms is zero, then $a/b$ is equals:

• A. $\frac{n-5}{6}$
• B. $\frac{n-4}{5}$
• C. $\frac{5}{n-4}$
• D. $\frac{6}{n-5}$

Answer 1

The correct option is B $\frac{n-4}{5}$

Acc. to Question

${}^m{C}_4{a}^{n-4}(-b{)}^4{+}^n{C}_5{a}^{n-5}(-b{)}^5=0$

solving we get

$a/b=\frac{n-4}{5}$