x² - 17x + 42 = 0
x1 + x2 = -b / a = -(-17) / 1 = 17
x1 × x2 = c / a = 42 / 1 = 42
x1 = 3
x2 = 14
x² - 7x - 30 = 0
x1 + x2 = -b / a = -(-7) / 1 = 7
x1 × x2 = c / a = -30 / 1 = -30
x1 = - 3
x2 = 10
x² + 5x + 3 = 0
D = b² - 4ac = 5² - 4 × 1 × 3 = 25 - 12 = 13
x = (-b ± корень из 13) / 2a
x1 = (-5 + корень из 13) / 2
x2 = (-5 - корень из 13) / 2
x² + px + 84 = 0
x1 + x2 = -p
x1 × x2 = 84
Если x1 = 7, то
7 + x2 = -p
7 × x2 = 84
x2 = 84 / 7
x2 = 12
7 + 12 = -p
p = - 19
(x + 5) / x² + 19x + 70
x² + 19x + 70 = 0
D = 361 - 4 × 70 = 361 - 280 = 81
x1 = (-19 + 9) / 2 = -10 / 2 = -5
x2 = (-19 - 9) / 2 = -28 / 2 = -14
(x + 5) / (x + 5) × (x + 14) = 1 / x + 14
