Imports System.IO
Imports System.Drawing.Drawing2D
 
Public Class Form1
 
    Public seitea, seiteb, seitec As Double
    Public summe As Single
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        seitea = Val(TextBox1.Text)
        seiteb = Val(TextBox2.Text)
        seitec = Val(TextBox3.Text)
        Call dreieck(seitea, seiteb, seitec)
    End Sub
    Function dreieck(ByVal seitea As Double, ByVal seiteb As Double, ByVal _
      seitec As Double) As Double
        summe = (seitea + seiteb + seitec) / 2
        TextBox4.Text = Math.Round(summe, 3)
        Return seitea
    End Function
    Private Sub Form1_Paint(ByVal sender As Object, ByVal e As _
      System.Windows.Forms.PaintEventArgs) Handles PictureBox1.Paint
        Dim g As Graphics = e.Graphics
        PictureBox1.BackColor = Color.Yellow
        g.DrawLine(Pens.Black, 100, 100, 200, 200)
        g.DrawLine(Pens.Black, summe, 100, 200, 200)
    End Sub
End Class

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Alle Angaben ohne Gewähr. Keine Haftung für Vorschläge, Tipps oder sonstige Hilfe, falls es schiefgeht, nur Zeit verschwendet oder man sonst nicht zufrieden ist

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As  _
      System.EventArgs) Handles btnDraw.Click
        seitea = Val(textbox1.Text)
        seiteb = Val(textbox2.Text)
        seitec = Val(textbox3.Text)
        Call dreieck(seitea, seiteb, seitec)
        Picturebox1.Invalidate()
    End Sub
 
    Sub dreieck(ByVal seitea As Double, ByVal seiteb As Double, ByVal _
        seitec As Double)
        summe = CSng((seitea + seiteb + seitec) / 2)
        TextBox4.Text = Math.Round(summe, 3)
        Return
    End Sub
 
    Private Sub PictureBox1_Paint(ByVal sender As Object, ByVal e As  _
      System.Windows.Forms.PaintEventArgs) Handles PictureBox1.Paint
        Dim g As Graphics = e.Graphics
        g.Clear(Color.Yellow)
        g.DrawLine(Pens.Black, 100, 100, 200, 200)
        g.DrawLine(Pens.Black, CInt(summe), 100, 200, 200)
    End Sub

Option Strict On : Option Explicit On
Public Class frmTriangle
 
    Dim pn As New Panel With _
        {.Parent = Me, .Top = 50, .Width = 400, .Height = 300}
    Dim pb As New PictureBox With _
        {.Parent = pn, .SizeMode = PictureBoxSizeMode.Zoom, .Dock = _
        DockStyle.Fill}
 
    Dim tba As New TextBox With _
        {.Parent = Me, .Top = 0, .Width = 80}
    Dim tbb As New TextBox With _
        {.Parent = Me, .Top = 0, .Left = 100, .Width = 80}
    Dim tbc As New TextBox With _
        {.Parent = Me, .Top = 0, .Left = 200, .Width = 80}
 
    Dim WithEvents btnDraw As New Button With _
       {.Parent = Me, .Left = 300, .Text = "Draw"}
 
    Dim pos1, pos2, pos3 As Point
 
    Private Sub frmTriangle_Load(sender As System.Object, _
        e As System.EventArgs) Handles MyBase.Load
 
        Me.Width = 400 : Me.Height = 400
        Me.FormBorderStyle = Windows.Forms.FormBorderStyle.Fixed3D
        Me.Text = "Seitenlängen a,b,c eingeben --   a + b => c"
        tba.Text = CStr(100) : tbb.Text = CStr(120) : tbc.Text = CStr(150)
    End Sub
 
 
    Private Sub btnDraw_Click(sender As Object, _
        e As System.EventArgs) Handles btnDraw.Click
 
        Dim a, b, c As Integer, ok As Boolean = True
        'Eingaben checken 
        If Not Integer.TryParse(tba.Text, a) Then Exit Sub
        If Not Integer.TryParse(tbb.Text, b) Then Exit Sub
        If Not Integer.TryParse(tbc.Text, c) Then Exit Sub
 
        'Kosinussatz anwenden
        Dim cosval As Double = (a * a - (b * b + c * c)) / (-2 * b * c)
        Dim alpha As Double = Math.Acos(cosval)
        'Definieren Seitenlängen ein Dreieck? 
        If Double.IsNaN(alpha) Then
            MsgBox("Ungeeignete Eingabe", MsgBoxStyle.Exclamation) : Exit Sub
        End If
 
        'Zwei Punkte festlegen
        pos1 = New Point(100, 100)
        pos2 = New Point(100 + CInt(b), 100)
 
        'Position des 3. Punktes gemäß obigem Winkel bestimmen 
        pos3 = New Point(pos1.X + CInt(Math.Cos(alpha) * c), _
                         pos1.Y + CInt(Math.Sin(alpha) * c))
 
        Dim xmax As Integer = CInt(Math.Max(pos2.X, pos3.X) + 50)
        Dim ymax As Integer = CInt(Math.Max(pos2.Y, pos3.Y) + 50)
 
        'Überprüfung der Punkte-Position: 
        'Euklidische Distanz zum dritten Punkt berechnen
        Dim diff1 As Integer = _
            CInt(Math.Sqrt((pos1.X - pos3.X) ^ 2 + (pos1.Y - pos3.Y) ^ 2))
        If Math.Abs(diff1 - c) > 2 Then ok = False
        Dim diff2 As Integer = _
            CInt(Math.Sqrt((pos2.X - pos3.X) ^ 2 + (pos2.Y - pos3.Y) ^ 2))
        If Math.Abs(diff2 - a) > 2 Then ok = False
 
        If Not ok Then
            MsgBox("Berechnungsfehler", MsgBoxStyle.Exclamation)
        Else
            Using bmp As New Bitmap(xmax, ymax), _
                gr As Graphics = Graphics.FromImage(bmp)
                gr.Clear(Color.White)
 
                'Dreieck Zeichnen
                gr.FillPolygon _
                 (New SolidBrush(Color.Red), New Point() {pos1, pos2, pos3})
 
                gr.DrawPolygon _
                  (New Pen(Color.Blue, 3), New Point() {pos1, pos2, pos3})
 
                'Dreieck anzeigen
                pb.Image = CType(bmp.Clone, Image)
            End Using
        End If
    End Sub
 
 End Class