initial ch06 commit
ex 6.1, 6.2, 6.3, 6.4
ex 6.5, 6.6, 6.7, 6.8
ex 6.9
before chapter 6.5
ex 6.10
chapter 6.6
initial ex 6.11
initial ex 6.11 revised
ex 6.11
ex 6.11 worksheet
ex 6.11 worksheet