12 Chapter 2, Second order linear equations. Section 2.4.2 Variation of parameters. Exercises page 124

 12.1 problem 1(a)
 12.2 problem 1(b)
 12.3 problem 1(c)
 12.4 problem 1(d)
 12.5 problem 1(e)
 12.6 problem 1(f)
 12.7 problem 2
 12.8 problem 3
 12.9 problem 7