27 Chapter 3 (Systems of differential equations). Section 20. The method of elimination. Exercises page 212

27.1 problem 776
27.2 problem 777
27.3 problem 778
27.4 problem 779
27.5 problem 780
27.6 problem 781
27.7 problem 782
27.8 problem 783
27.9 problem 784
27.10 problem 785
27.11 problem 786