Texas Instruments TI-73 Explorer Manual Book page 58

52
Chapter 2: Math Operations
³
From a group of 4 items (ABCD), how many ways can you
select 2 of the items if the order does not matter?
Find 4 2.
nCr
:
b
2
ABCD
! ! 1 " " 5 5
Returns the factorial of value. value can be an integer or list of
integers between 0 and 69. By definition, 0! = 1.
Factorials are similar to permutations because the order
DOES matter. You can think of 4! as the total number of ways
that 4 items can be arranged.
value
!
³ Find 4!
:
b
4!
ABCD
1 " "
4
combinations of
4 items taken
2 at a time
1 " "
4
ABCD
BACD
CABD
DABC
4
AB AC
BA BC
CA CB
DA DB
5
24 possible arrangements
ABDC ACBD ACDB
BADC BCAD BCDA
CADB CBAD CBDA
DACB DBAC DBCA
AB and BA count
as only one
AD
combination.
BD
CD
DC
4! = 4Q3Q2Q1
ADBC ADCB
BDCA BDAC
CDAB CDBA
DCAB DCBA