Question

If the co-efficients of x⁷ and x⁸ in $\left( 2 + \frac{x}{3} \right)^{n}$ are equal, then n is

• A. 56
• B. 55
• C. 45
• D. 15

Answer 1

Answer: (B)

55

Answer 2

Here $nC_{7}\frac{2^{n - 7}}{3^{7}} =^{n}C_{8}\frac{2^{n - 8}}{3^{8}}$ ⇒ n = 55.