Proofs for Sets: Expert Help and Tips for Math Homework

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Homework Statement

hopefully the writing is readable:
http://i.imgur.com/VJ8vN.jpg

All three if possible.

Homework Equations

none

The Attempt at a Solution

To be completely honest, I missed that whole week of lectures due to personal problems and I've had no chance to talk to an instructor or a classmate for some notes before this is due. I'm perfectly fine with just tiny tips here and there.

http://i.imgur.com/4AknY.jpg

My lame attempt at solving the first.

 

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what have I done?

 

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I was able to figure out the first two I believe. But at the moment I seem to be stuck on the 3rd. Would it be alright if I was to list all the options a and b have in order for ab=4 and then write that we can see they all work?

This is for a Math 273 course.