d = |2a+ 3(a-1)+3|/√[a²+(a-1)²] = 5|a|/√[a²+(a-1)²] ≥ 3
25a² ≥ 9[a²+(a-1)²]
25a² ≥ 9(2a² - 2a +1)
7a² + 18a-9 ≥ 0
(7a - 3)(a+3) ≥0
a ≥ 3/7 或 a < -3
d = |2a+ 3(a-1)+3|/√[a²+(a-1)²] = 5|a|/√[a²+(a-1)²] ≥ 3
25a² ≥ 9[a²+(a-1)²]
25a² ≥ 9(2a² - 2a +1)
7a² + 18a-9 ≥ 0
(7a - 3)(a+3) ≥0
a ≥ 3/7 或 a < -3