Question

Ajutor!!Combinari de 2 luate cate n+2.
n=n+2
k=2​

Answer 1

$\it C_{n+2}^2=21 \Rightarrow \dfrac{(n+2)!}{2!(n+2-2)!}=21 \Rightarrow\dfrac{n!(n+1)(n+2)}{2\cdot n!}=21 \Rightarrow\\ \\ \Rightarrow (n+1)(n+2)=42=6\cdot7 \Rightarrow n+1=6 \Rightarrow n=5$