Question

已知两条直线L1:(a-1)x+2y+1=0，L2:x+ay+3=0平行，则a=?

A.-1 B.2 C.0或-2 D.-1或2

Answer 1

(a-1)x+2y+1=0, 2y=(1-a)x-1, y=(1-a)/2x-1/2
x+ay+3=0
(1) a=0, x+3=0, x=-3
L1:(0-1)(-3)+2y+1=0, 2y+4=0, y=-2
L1 和 L2 垂直
(2) a<>0, ay=-x-3, y=-1/ax-3/a
L1与L2平行
(1-a)/2=-1/a, a(a-1)=2, a²-a-2=0, (a+1)(a-2)=0
a=-1 , a=2