What does the term "compute" mean in relation to a problem I have
in which, there is the number "7" in an expontial form set to the
lower front of a letter,in this case,a "C" and followed by the
number "2",again in an expotinal form placed in a lower setting?
It looks somthing like example below:
7C2
That means:
"The number of combinations that can be gotten from 7 things if
you take 2 at a time"
It asks this question:
How many possible sets of 2 things each can you make from a set
of 7 things?
OR
If you have a set of 7 things, say {a,b,c,d,e,f,g}, then how many
different sets of only 2 each can you make from the set of 7?
We can make all these sets of 2:
1. {a,b}
2. {a,c}
3. {a,d}
4. {a,e}
5. {a,f}
6. {a,g}
7. {b,c}
8. {b,d}
9. {b,e}
10. {b,f}
11. {b,g}
12. {c,d}
13. {c,e}
14. {c,f}
15. (c,g}
16. {d,e}
17, {d,f}
18. {d,g}
19. {e,f}
20. {e,g}
21. {f,g}
So there are 21 in all. In general there are
too many to list them all out like I did above,
so there is a formula that gives the number
without having to list them like the above. It is
n!
nCr = ----------
r!(n-r)!
In the case where the big set has 7 and you
want to make little sets of 2, instead of
listing them all, you just calculate:
7!
7C2 = -------- =
2!(7-2)!
The exclamation point means to multiply the
number before it by all positive integer
factors below it (it's called "factorial"):
7!
---- =
2!5!
7·6·5·4·3·2·1
---------------- =
(2·1)(5·4·3·2·1)
Cancel like factors in the
top and bottom
3 1 1 1 1 1
7·6·5·4·3·2·1
---------------- =
(2·1)(5·4·3·2·1)
1 1 1 1 1 1
7·3 = 21
So we can get that there are 21 combinations possible
when 7 things are taken 2 at a time.
Edwin