Для прямоугольника
program Diagonali;
var a,b,c:array[1..2] of integer;
AB,BC:real;
function Diagonal(a,b:real):real;
begin
Diagonal:= sqrt(sqr(a)+sqr(b));
end;
BEGIN
Writeln('Vvedite koordinatbl tochek');
Read(a[1],a[2],b[1],b[2],c[1],c[2]);
AB:= sqrt( sqr((b[1]-a[1]) + sqr(b[2]-a[2])) );
BC:= sqrt( sqr((C[1]-B[1]) + sqr(c[2]-b[2])) );
writeln(AB);
writeln('Dlinna diagonali = ',Diagonal(AB,BC));
readln;
END.