11 Chapter 18, The variation of constants formula. Exercises page 168

11.1 problem 18.1 (i)
11.2 problem 18.1 (ii)
11.3 problem 18.1 (iii)
11.4 problem 18.1 (iv)
11.5 problem 18.1 (v)
11.6 problem 18.1 (vi)