# Unions and Intersections

• Feb 25th 2013, 06:17 PM
zhengcl86
Unions and Intersections
Union and Intersections
Let A= {x|x is a prime factor of 720}
Let B= {x|x is a prime factor of 330}

Find
a) A intersect B
b) A union B
c) A - B

i don't know how to get the answers, is there an easy way to do this kind of problem?
• Feb 25th 2013, 06:20 PM
emakarov
Re: Unions and Intersections
List prime factors of 720 and 330 (without repetition). For a), write factors appearing in both groups. For b), join the two groups (again, write each number only once). For c), write factors of 720 that are not factors of 330.
• Feb 25th 2013, 06:45 PM
zhengcl86
Re: Unions and Intersections
so
A intersect B = 2,3,5
A union B = 2,3,5,11
A - B = 11

is that correct?
• Feb 25th 2013, 06:53 PM
emakarov
Re: Unions and Intersections
Intersection and union are correct. A - B is empty, but B - A = {11} (set elements are written between curly braces; 11 itself is not a set).
• Feb 25th 2013, 07:42 PM
zhengcl86
Re: Unions and Intersections
hmm somewhat understandable thank you for the reply