c# - Transcribing a polygon on a circle -

i try inscribe diagonals of decagon inside circle

like

in c# approach creating circle

e.graphics.drawellipse(mypen, 0, 0, 100, 100); 

and draw lines inside using

 e.graphics.drawline(mypen, 20, 5, 50, 50); 

after draw decagon polygon.

currently im stuck @ how divide circle 10 parts/ finding correct coordiantes of points on circumference of circles because im not in math, want know how know next point in circumference of circle size of circle indicated above.
, want ask better approach problem.

thank :)

just grits , shins, here's generic implementation inscribe x-sided polygon rectangle pass it. note in approach i'm not calculating absolute points. instead, translating origin, rotating surface, , drawing lines respect origin using fixed length , angle. repeated in loop achieve end result below, , similar commanding turtle in logo:

public partial class form1 : form {      picturebox pb = new picturebox();     numericupdown nud = new numericupdown();      public form1()     {         initializecomponent();          this.text = "inscribed polygon demo";          tablelayoutpanel tlp = new tablelayoutpanel();         tlp.rowcount = 2;         tlp.rowstyles.clear();         tlp.rowstyles.add(new rowstyle(sizetype.autosize));         tlp.rowstyles.add(new rowstyle(sizetype.percent, 100));         tlp.columncount = 2;         tlp.columnstyles.clear();         tlp.columnstyles.add(new columnstyle(sizetype.autosize));         tlp.columnstyles.add(new columnstyle(sizetype.autosize));         tlp.dock = dockstyle.fill;         this.controls.add(tlp);          label lbl = new label();         lbl.text = "number of sides:";         lbl.textalign = contentalignment.middleright;         tlp.controls.add(lbl, 0, 0);          nud.minimum = 3;         nud.maximum = 20;         nud.autosize = true;         nud.valuechanged += new eventhandler(nud_valuechanged);         tlp.controls.add(nud, 1, 0);          pb.dock = dockstyle.fill;         pb.paint += new painteventhandler(pb_paint);         pb.sizechanged += new eventhandler(pb_sizechanged);         tlp.setcolumnspan(pb, 2);         tlp.controls.add(pb, 0, 1);     }      void nud_valuechanged(object sender, eventargs e)     {         pb.refresh();     }      void pb_sizechanged(object sender, eventargs e)     {         pb.refresh();     }      void pb_paint(object sender, painteventargs e)     {         // make circle centered , 90% of picturebox size:         int radius = (int)((double)math.min(pb.clientrectangle.width, pb.clientrectangle.height) / (double)2.0 * (double).9);         point center = new point((int)((double)pb.clientrectangle.width / (double)2.0), (int)((double)pb.clientrectangle.height / (double)2.0));         rectangle rc = new rectangle(center, new size(1, 1));         rc.inflate(radius, radius);          inscribepolygon(e.graphics, rc, (int)nud.value);     }      private void inscribepolygon(graphics g, rectangle rc, int numsides)     {         if (numsides < 3)             throw new exception("number of sides must greater or equal 3!");          float radius = (float)((double)math.min(rc.width, rc.height) / 2.0);         pointf center = new pointf((float)(rc.location.x + rc.width / 2.0), (float)(rc.location.y + rc.height / 2.0));         rectanglef rcf = new rectanglef(center, new sizef(1, 1));         rcf.inflate(radius, radius);         g.drawellipse(pens.black, rcf);          float sides = (float)numsides;         float exteriorangle = (float)360 / sides;         float interiorangle = (sides - (float)2) / sides * (float)180;         float sidelength = (float)2 * radius * (float)math.sin(math.pi / (double)sides);         (int = 1; <= sides; i++)         {             g.resettransform();             g.translatetransform(center.x, center.y);             g.rotatetransform((i - 1) * exteriorangle);             g.drawline(pens.black, new pointf(0, 0), new pointf(0, -radius));             g.translatetransform(0, -radius);             g.rotatetransform(180 - interiorangle / 2);             g.drawline(pens.black, new pointf(0, 0), new pointf(0, -sidelength));         }     }  } 

i got formula length of side here @ regular polygon calculator.