completed subsection 9.1.1
completed subsection 9.1.2
completed subsection 9.1.3
completed subsection 9.2.1
completed subsection 9.2.2
completed section 9.2
completed subsection 9.3.1
completed subsection 9.3.2
completed subsection 9.3.3
completed subsection 9.4.1
completed subsection 9.4.2
completed excersises for chapter 9