The circumcircle of a triangle is a circle that passes through all of the vertices of the triangle. The
circumcircle of a polygon is a circle that passes through all of the vertices of the polygon.
The circumcenter of a triangle is the center of the circumcircle of the triangle.
The circumcenter's position depends on the type of triangle
• If and only if a triangle is acute (all angles smaller than a right angle), the circumcenter lies
inside the triangle
• If and only if it is obtuse (has one angle bigger than a right angle), the circumcenter lies outside
• If and only if it is a right triangle, the circumcenter lies on one of its sides (namely, the
hypotenuse). This is one form of Thales' theorem.
Inscribed Circle/Incircle:
The incircle is the inscribed circle of a triangle ABC, i.e., the unique circle that is tangent to each of
the triangle's three sides. The center I of the incircle is called the
incenter, and the radius r of the
circle is called the inradius.
The incenter is the point of concurrence of the triangle's angle bisectors. In addition, the points MA,
MB and MC of intersection of the incircle with the sides of triangle ABC
are the polygon vertices of
the pedal triangle taking the incenter as the pedal point (c.f.
tangential triangle). This triangle is
called the contact triangle.