add ex5-1
add ex5-2
add ex5-3
add ex5-4
add ex5-5
add ex5-7
add ex5-8
add ex5-9
add ex5-10
add commonts on chapter 3, and modify ex3-3
fix bugs in ex3-3
add comments on chapter04
add comments on chapter 5
add comments
add ex5-11
add ex5-12
add ex5-13
add comments on ex5-13
add ex5-14
add ex5-15