Detail the proof of  3.1.md
Detail the proof of  3.1.md

I have detail some of proofs in 3.1.md.
Below is a list of answers modified by me:
1. Exercise 3.1-4
2. Exercise 3.1-5
3. Exercise 3.1-7

semsevens/CLRS,Jingru/CLRS,semsevens/CLRS,Jingru/CLRS,gzc/CLRS,gzc/CLRS,Jingru/CLRS,semsevens/CLRS,gzc/CLRS