81.3 Chapter 4. Homogeneous differential equations.

81.3.1 problem 4-6
81.3.2 problem 4-7
81.3.3 problem 4-8
81.3.4 problem 4-9
81.3.5 problem 4-10 (ii)
81.3.6 problem 4-11
81.3.7 problem 4-12
81.3.8 problem 4-13
81.3.9 problem 4-14
81.3.10 problem 4-15
81.3.11 problem 4-16
81.3.12 problem 4-17
81.3.13 problem 4-18
81.3.14 problem 4-19
81.3.15 problem 4-20