✔ 最佳答案
1.
L1的斜率 = -3/(a - 2)
L2的斜率 = -a/3
由於 L1⊥L2,所以 (L1的斜率) ´ (L2的斜率) = -1
[-3/(a - 2)] ´(-a/3) = -1
a/(a - 2) = -1
a - 2 = -a
2a = 2
a = 1
= = = = =
2.
f(x) = ax² + bx + c
f(x) = a[x² + (b/a)x] + c
f(x) = a[x² + (b/a)x + (b/2a)²] + c - a(b/2a)²
f(x) = a[x + (b/2a)]² + c - a(b/2a)²
由於 [x + (b/2a)]² ≥0,故f(x)有最小值時:
[x +(b/2a)]² = 0
x =-b/2a
所以 -b/2a = 2
b = -4a ...... [1]
f(2) = 1 :
a(2)² + b(2) + c = 1
4a + 2b + c = 1 ...... [2]
f(1) = 2
a(1)² + b(1) + c = 2
a + b + c = 2 ...... [3]
[2] - [3] :
3a + b = -1 ...... [4]
把 [1] 代入 [2] 中:
3a + (-4a) = -1
a = 1
把 a = 1 代入 [1] 中:
b = -4(1)
b = -4
把 a = 1 及 b = -4 代入 [3] 中:
(1) + (-4) + c = 2
c = 5
f(x) = x² - 4x + 5
= = = = =
3.
AB 的斜率 = (2 + 2)/(3 + 1) = 1
AB 的垂直平分線斜率 = -1/1 = -1
AB 的中點 = ((-1 + 3)/2, (-2 + 2)/2) = (1, 0)
AB 的垂直平分線方程式:
y - 0 = -1(x - 1)
x + y - 1 = 0