Exercises of plane geometry based on:
Easy →   c1 →   c2 →   c3 →   c4 →   c5 →   c6 →   c7 →   c8 →   c9
Normal →   b1 →   b2 →   b3 →   b4 →   b5 →   b6 →   b7 →   b8
Difficult →   a1 →   a2 →   a3 →   a4 →   a5 →   a6