Chapter 11. Variation of parameters and other methods. Exercises at page 177

89.25.1 problem 1
89.25.2 problem 2
89.25.3 problem 3
89.25.4 problem 4
89.25.5 problem 6
89.25.6 problem 7
89.25.7 problem 8
89.25.8 problem 9
89.25.9 problem 10