Algebraic property of a set

If A A and B B are subsets of U = { 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 } U=\{1, 2, 3, 4, 5, 6, 7, 8, 9\} such that A c B c = { 1 , 9 } , A B = { 2 } , A c B = { 4 , 6 , 8 } , { A }^{ c }\cap { B }^{ { c } }=\{1, 9\}, A\cap B =\{2\}, { A }^{ c }\cap B=\{4, 6, 8\}, what is the set A ? A?

{ 2 , 3 , 5 , 7 } \{2, 3, 5, 7\} { 3 , 4 , 9 } \{3, 4, 9\} { 1 , 2 , 6 , 7 } \{1, 2, 6, 7\} { 5 , 8 } \{5, 8\}

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3 solutions

Zandra Vinegar Staff
Oct 18, 2015

Line by line...

A A and B B are subsets of U = { 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 } U=\{1, 2, 3, 4, 5, 6, 7, 8, 9\} such that

A c B c = { 1 , 9 } { A }^{ c }\cap { B }^{ { c } }=\{1, 9\} --- means that neither 9 nor 1 is in A or B, and everything except 9 and 1 are in at least one of A or B.

A B = { 2 } A\cap B =\{2\} --- means that 2 is the only thing in both A and B, leaving 3, 4, 5, 6, 7, and 8 to be in one of A or B but not both.

A c B = { 4 , 6 , 8 } , { A }^{ c }\cap B=\{4, 6, 8\}, --- means that 4, 6, and 8 are the things in B but not A, leaving 3, 5, and 7 to be in A but not B.

So, in total, 2, 3, 5, and 7 are in A.

Eli Child
May 15, 2017

Solution by answer choices:

You only need one of the three clues. When it's learned 4, 6, and 8 are all part of the complement of A in the third clue, three answer choices are ruled out.

Denis Husadzic
Jan 27, 2018

( A c B ) ( A c B c ) = A c ( B B c ) = A c U = A c A c = { 1 , 9 } { 4 , 6 , 8 } A = { 2 , 3 , 5 , 7 } (A^c\cap B)\cup (A^c\cap B^c) = A^c\cap (B\cup B^c) = A^c\cap U = A^c \implies A^c = \{ 1,9\}\cup \{4,6,8\} \implies A= \{2,3,5,7\}

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