#### Answer

{4}

#### Work Step by Step

A ∩ B
∩ means intersect when talking about sets.
A ∩ B means to find the intersection of the elements of set A and set B. By intersection we mean those elements that are in A and in B. The elements must be in both sets to be in the intersection.
A={1,3,4,5,9}
B={2,4,6,7,8}
You can find the intersection by comparing each element in A with each element in B.
1 is in A, is it in B? No so drop it. You can continue checking the rest of the elements. Anything that remains will be in both sets (the intersection).
{4}
4 is the only number left, the only number in both sets, so A ∩ B ={4}.