1. x^{2} - 9x + 20 = 0
x1, 2 = ( 9 +- \sqrt{81 - 80} ) / 2; x1 = 4; x2 = 5
2. x^{2} - 9x = 0 ; x = 9
3. x^{2} - 9x + 20 = 12
x^{2} - 9x + 6 = 0
x1, 2 = ( 9 +- \sqrt{81 - 24} ) / 2; x1 \approx 0.73; x2 \approx 8.27
4. x^{2} - 9x - 10 = 0
x1, 2 = ( 9 +- \sqrt{81 + 40} ) / 2; x1 = 10; x2 = 5