81.2 Chapter II. Change of variable. Exercises at page 20

81.2.1 problem 1
81.2.2 problem 2
81.2.3 problem 3
81.2.4 problem 4
81.2.5 problem 5
81.2.6 problem 6
81.2.7 problem 7
81.2.8 problem 8
81.2.9 problem 9
81.2.10 problem 10