Question

If nC₁₂ = nC₉ find 21C_n

Answer 1

Given nC₁₂ = nC₉

We have nC_x = nC_y 

⇒ x = y or x + y = n

nC₁₂ = nC₉

⇒ 12 + 9 = n

⇒ n = 21

⇒ 21C_n = 21C₂₁ = 1