✔ 最佳答案
x² + ax + (a - 1) = 0
兩根之和:α + β = a ...... [1]
兩根之積:αβ = a - 1 ...... [2]
[1]² :
(α + β)² = a²
α² + 2αβ + β² = a² ...... [3]
[2] × 4 :
4αβ = 4a - 4 ...... [4]
[3] - [4] :
α² - 2αβ + β² = a² - 4a + 4
(|α - β|)² = (a - 2)²
因為 |α - β| = 4, then :
(a - 2)² = 4²
(a - 2)² = 16
a - 2 = 4 或 a - 2 = -4
a = 6 或 a = -2