1.

If two sets A and B have 99 elements in common, then the number of elements common to each of the sets A×B and B×A are

Answer»

To find A × B we take one element from set A and one from set B. 

Given that 99 elements are common to both set A and set B.

Suppose these common elements are N1​, N2​, N3​,... N99​.

Select an ordered pair for A×B such that both are selected out of these common elements.

Examples: (N1​,N2​),(N3​,N5​)

All these will also be elements of B × A. 

Hence number of elements common to A × B and B × A is 99 × 99 = 992 ( first element in ordered pair can be selected in 99 ways; second element can also be selected in 99 ways)

n[(A×B)∩(B×A)] = n[(A∩B)∩(B∩A)] = (99)(99) = 992



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