ch5 31-36
update 3v
Page 55-59 of chp7 done by Young
Merge pull request #21 from yan9yu/gh-pages

Page 55-59 of chp7 done by Young
Merge pull request #19 from hanlu-chen/develop

ch5 31-36
Merge pull request #20 from dada889/gh-pages

update 3v
Merge branch 'gh-pages'