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