2x-4y=0 => y = x/2 代入 3x-2y+5=0 =》2x+5=0
=> 2x-4y=0 与 3x-2y+5=0 的交点 (-5/2,-5/4)
与L1垂直的直线,斜率 k= -1/(1/2) = - 2
所求直线：y - (-5/4) = -2 [ x - (-5/2) ]
即 y + 5/4 = - 2x - 5
2x + y + 25/4 = 0