Pascal's Triangle

Hello AnxiousLlama,

I am problably too late and I'm not sure, but still I give a try:

$\binom{n-1}{r-1}+\binom{n-1}{r}$ 

order the terms using the commutative law

$\binom{n-1}{r}+\binom{n-1}{r-1}$

simplify with the Pascal's triangle $\binom{n}{k}+\binom{n}{k-1}=\binom{n+1}{k}$ to 

$\binom{n}{r}$ so it's true.

Straight