求经过线段中点的法线的方程？

[yyyrrr51]

已知两点座标为(x1,x2),(y1,y2)的线段，求经过这条线段中心点的垂直线(法线)的方程

[SYSSZ]

Private Sub Form_Load()
Dim x1, y1, x2, y2, x, y As Double
Form1.Show
Form1.Width = Form1.Height
Form1.AutoRedraw = True
Form1.DrawWidth = 2
Form1.Scale (0, 0)-(50, 50)
 x1 = 1: y1 = 1: x2 = 40: y2 = 45
Line (x1, y1)-(x2, y2)
x = 2
Y3 = ((x2 - x1) * (2 * x - x1 - x2)) / (2 * y1 - 2 * y2) + (y1 + y2) / 2
Debug.Print Y3
 x = 50
 y4 = ((x2 - x1) * (2 * x - x1 - x2)) / (2 * y1 - 2 * y2) + (y1 + y2) / 2
Line (0, Y3)-(50, y4)
End Sub

[红叶哥]

Dim X As Double, Y As Double
Y = (X1 - Y1) / (X2 - Y2) * X + X1 * (Y2 - Y1) / (X1 - Y1)