\choose - Used to draw notation commonly used for binomial coefficients;
{ <subformula1> \choose <subformula2> }
\chi command draws chi symbol.
\displaystyle n+1 \choose k+2
\displaystyle {n+1 \choose k+2}
n+1 \choose k+2
$n+1 \choose k+2$
$1+{n \choose 2}+k$