A . MATHS - 直線

2x - 3y + 5 + k(x - 4y + 5) = 0
(2 + k)x - (3 + 4k)y + 5 + 5k = 0
slope = (2 + k)/(3 + 4k)
L3 : 2x - 3 = 0 is vertical (垂直)
成 45 的交角, 所引直線 slope = 1 or -1
(2 + k)/(3 + 4k) = 1 or (2 + k)/(3 + 4k) = -1
2 + k = 3 + 4k or 2 + k = -3 - 4k
k = -1/3 or k = -1
when k = -1/3, (2 - 1/3)x - (3 - 4/3)y + 5 - 5/3 = 0
x - y + 2 = 0
when k = -1, (2 - 1)x - (3 - 4)y + 5 - 5 = 0
x + y = 0