Selections (VCE SSCE Mathematical Methods): Flashcards

Arrangements: order matters; Selections: order doesn't matter

Combinations

Number of ways to choose $r$ objects from $n$ (order unimportant)

$^nC_r = \frac{n!}{r!(n-r)!}$

Calculate selections for each group, then multiply

$2^n$

Always 1

Sum of the two numbers directly above it

$^nC_r = ^{n-1}C_{r-1} + ^{n-1}C_r$ for $0 < r < n$

$\binom{n}{r}$