Trekantens midtnormaler skæres i et punkt. Midtnormalen er en linje gennem en sides midtpunkt, som er vinkelret på siden. Du kan flytte trekantens hjørner, A, B og C, med musen. Læg mærke til, at de tre midtnormaler altid har et fælles punkt, P. Dette punkt er centrum for trekantens omskrevne cirkel.
Figure figure = Position [0,0] Size[x,y] Origin[x/3,y/2] Unit x/12 Color "white";
//Axes axes = Color "black";
//Grid grid = Color "blue";
//Units units = Color "black";
Point A = [-2,-1] Free Size 2.5 Color "red";
Point B = [5,-1] Free Size 2.5 Color "red";
Point C = [0,3] Free Size 2.5 Color "red";
Label laba = "A" At A Offset [10,-5] Color "black";
Label labb = "B" At B Offset [10,-5] Color "black";
Label labc = "C" At C Offset [10,-5] Color "black";
Line ab = Start A Dir B-A Size 1 Color "black";
Line bc = Start B Dir C-B Size 1 Color "black";
Line ca = Start C Dir A-C Size 1 Color "black";
Variable ma = (B+C)/2;
Variable mb = (A+C)/2;
Variable mc = (B+A)/2;
Line mna = Start ma Dir (B-C)^ Infinite Size 1 Color "blue";
Line mnb = Start mb Dir (A-C)^ Infinite Size 1 Color "blue";
Line mnc = Start mc Dir (A-B)^ Infinite Size 1 Color "blue";
Variable orientation = if (B-A)*(C-A)^ < 0 then 0 else pi endif;
RightAngle raa = At ma From angle(C-B)+orientation To angle(C-B)+pi/2+orientation Size 0.3 Color "black";
RightAngle rab = At mb From angle(A-C)+orientation To angle(A-C)+pi/2+orientation Size 0.3 Color "black";
RightAngle rac = At mc From angle(B-A)+orientation To angle(B-A)+pi/2+orientation Size 0.3 Color "black";
Point P = intersect(ma,ma+(B-C)^,mb,mb+(A-C)^) Size 2.5 Color "blue";
Label labp = "P" At P Offset [10,-5] Color "black";
Circle circle = Center P Radius length(A-P) Size 1 Color "blue";