add ch12.3.1
1 correct a format issure in 12.3.1 and add 12.3.2
add 12.3.3
add 12.3.5
add 12.7.1.png
add 12.7.1
add 12.7.2
add 12.7.2.png
add 12.7.2.png for better illusrating the answer
add 12.7.3
Merge pull request #33 from fangang190/master

Add answers for 12.3.1 12.3.2