Answer:
[tex]\fbox{\begin{minipage}{14em}Number of subsets: 16\\Number of proper subsets: 15\end{minipage}}[/tex]
Step-by-step explanation:
Given:
The set A = {5, 13, 17, 20}
Question:
Find the number of subsets of A
Find the number of proper subsets of A
Simple solution by counting:
Subset of A that has 0 element:
{∅} - 1 set
Subset of A that has 1 element:
{5}, {13}, {17}, {20} - 4 sets
Subset of A that has 2 elements:
{5, 13}, {5, 17}, {5, 20}, {13, 17}, {13, 20}, {17, 20} - 6 sets
Subset of A that has 3 elements:
{5, 13, 17}, {5, 13, 20}, {5, 17, 20}, {13, 17, 20} - 4 sets
Subset of A that has 4 elements:
{5, 13, 17, 20} - 1 set
In total, the number of subsets of A: N = 1 + 4 + 6 + 4 + 1 = 16
The number of proper subsets (all of subsets, except subset which is equal to original set A): N = 16 - 1 = 15
Key-point:
The counting method might be used for finding the number of subsets when the original set contains few elements.
The question is that, for a set that contains many elements, how to find out the number of subsets?
The answer is that: there is a fix formula to calculate the total number ([tex]N[/tex]) of subsets of a set containing [tex]n[/tex] elements: N = [tex]2^{n}[/tex]
With original set A = {5, 13, 17, 20}, there are 4 elements belonged to A.
=> Number of subsets of A: N = [tex]2^{4} = 16[/tex]
(same result as using counting method)
Brief proof of formula: N = [tex]2^{n}[/tex]
Each element of original set is considered in 2 status: existed or not.
If existed => fill that element in.
If not => leave empty.
For i.e.: empty subset means that all elements are selected as not existed, subset with 1 element means that all elements are selected as not existed, except 1 element, ... and so on.
=> From the point of view of a permutation problem, for each element in original set, there are 2 ways to select: existed or not. There are [tex]n[/tex] elements in total. => There are [tex]2^n}[/tex] ways to select, or in other words, there are [tex]2^{n}[/tex] subsets.
Hope this helps!
:)
