用配方法解
x² + bx + c = 0
x² + bx = - c
x² + bx + b²/4 = b²/4 - c
(x+ b/2)² = (b² - 4c) / 4
x + b/2 = ±1/2 √(b² - 4c)
x = [ -b ±√(b² - 4c) ] / 2