Rotating line with pythagoras formula - vb.net

Question

I want to rotate a line but i don't want to use radians or pi.I created an algorithm (if i can call it like that).I use pythagoras formula with two functions and two timers to rotate the line ,but i think there's an error because it rotate faster after 180 degrees rotation.Why?Here's my code:
Public Class Form1
Private Structure poi
Dim x As Integer
Dim y As Integer
End Structure
Dim x1 As Integer, y1 As Integer
Dim x As Integer, y As Integer
Dim val As Integer
Dim obxy As poi
Dim moupos As poi
Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick
Call calcxy(val, 100)
x = obxy.x
y = obxy.y
val = val - 1
Refresh()
If -val > 100 Then
Timer1.Enabled = False
Timer2.Enabled = True
End If
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
If Button1.Text = "Start" Then
Timer1.Enabled = True
Button1.Text = "Stop"
Else
Timer1.Enabled = False
Timer2.Enabled = False
Button1.Text = "Start"
End If
End Sub
Sub calcxy(choice As Integer, radius As Integer)
Dim yval As Integer
yval = Math.Sqrt(radius ^ 2 - choice ^ 2)
obxy.x = moupos.x - choice
obxy.y = moupos.y - yval
End Sub
Sub recalcxy(choice As Integer, radius As Integer)
Dim yval As Integer
yval = Math.Sqrt(radius ^ 2 - choice ^ 2)
obxy.x = moupos.x - choice
obxy.y = moupos.y + yval
End Sub
Private Sub Form1_MouseClick(sender As Object, e As MouseEventArgs) Handles Me.MouseClick
moupos.x = e.X
moupos.y = e.Y
End Sub
Private Sub Form1_Paint(sender As System.Object, e As System.Windows.Forms.PaintEventArgs) Handles MyBase.Paint
Dim pen As New System.Drawing.Pen(System.Drawing.Color.Black, 1)
e.Graphics.DrawLine(pen, moupos.x, moupos.y, x, y)
End Sub
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
val = 100
End Sub
Private Sub Timer2_Tick(sender As Object, e As EventArgs) Handles Timer2.Tick
val = val + 1
Call recalcxy(val, 100)
x = obxy.x
y = obxy.y
Refresh()
If val + 1 > 100 Then
Timer2.Enabled = False
Timer1.Enabled = True
End If
End Sub
End Class