Inscribed angle
In geometry, an inscribed angle is the angle formed in the interior of a circle when two chords intersect on the circle. It can also be defined as the angle subtended at a point on the circle by two given points on the circle. Equivalently, an inscribed angle is defined by two chords of the circle sharing an endpoint. The inscribed angle theorem relates the measure of an inscribed angle to that of the central angle subtending the same arc. The inscribed angle theorem appears as Proposition 20 on Book 3 of Euclid's "Elements". Theorem. Statement. The  inscribed angle theorem states that an angle "θ" inscribed in a circle is half of the central angle 2"θ" that subtends the same arc on the circle. Therefore, the angle does not change as its vertex is moved to different positions on the circle. Proof. Inscribed angles where one chord is a diameter. Let "O" be the center of a circle, as in the diagram at right. Choose two points on the circle, and call them "V" and "A". Draw line "VO" and extended past "O" so that it intersects the circle at point "B" which is diametrically opposite the  point "V". Draw an angle whose vertex is point "V" and whose sides pass through points "A" and "B". Draw line "OA". Angle "BOA" is a central angle; call it "θ". Lines "OV" and "OA" are both radii of the circle, so they have equal lengths. Therefore, triangle "VOA" is isosceles, so angle "BVA" (the inscribed angle) and angle "VAO" are equal; let each of them be denoted as "ψ". Angles "BOA" and "AOV" add up to 180°, since line "VB" passing through "O" is a straight line. Therefore, angle "AOV" measures 180° − "θ". It is known  that the three angles of a triangle add up to 180°, and the three angles of triangle "VOA" are: Therefore, Subtract from both sides, where "θ" is the central angle subtending arc "AB" and "ψ" is the inscribed angle subtending arc "AB". Inscribed angles with the center of the circle in their interior. Given a circle whose center is point "O", choose three points "V", "C", and "D" on the circle. Draw lines "VC" and "VD": angle "DVC" is an inscribed angle. Now draw line "VO" and extend it past point "O" so that it intersects the circle at  point "E". Angle "DVC" subtends arc "DC" on the circle. Suppose this arc includes point "E" within it. Point "E" is diametrically opposite to point "V". Angles "DVE" and "EVC" are also inscribed angles, but both of these angles have one side which passes through the center of the circle, therefore the theorem from the above Part 1 can be applied to them. Therefore, then let so that Draw lines "OC" and "OD". Angle "DOC" is a central angle, but so are angles "DOE" and "EOC", and Let so that From Part One we know that formula_14 and that formula_15 . Combining these results with equation (2) yields therefore, by equation (1), Inscribed angles with the center of the circle in their exterior. The previous case can be extended to cover the case where the measure of the inscribed angle is the "difference" between two inscribed angles as discussed in the first part of this proof. Given a circle whose center is point "O", choose three points "V", "C", and "D" on the circle. Draw lines "VC" and "VD": angle "DVC" is an inscribed angle. Now draw line "VO" and extend it past point "O" so that it intersects the circle  at point "E". Angle "DVC" subtends arc "DC" on the circle. Suppose this arc does not include point "E" within it. Point "E" is diametrically opposite to point "V". Angles "EVD" and "EVC" are also inscribed angles, but both of these angles have one side which passes through the center of the circle, therefore the theorem from the above Part 1 can be applied to them. Therefore, then let so that Draw lines "OC" and "OD". Angle "DOC" is a central angle, but so are angles "EOD" and "EOC", and Let so that From Part One we know that formula_14  and that formula_15. Combining these results with equation (4) yields therefore, by equation (3), Corollary. By a similar argument, the angle between a chord and the tangent line at one of its intersection points equals half of the central angle subtended by the chord. See also Tangent lines to circles. Applications. The inscribed angle theorem is used in many proofs of elementary Euclidean geometry of the plane. A special case of the theorem is Thales' theorem, which states that the angle subtended by a diameter is always 90°, i.e., a right angle. As a consequence of the theorem, opposite  angles of cyclic quadrilaterals sum to 180°; conversely, any quadrilateral for which this is true can be inscribed in a circle. As another example, the inscribed angle theorem is the basis for several theorems related to the power of a point with respect to a circle. Further, it allows one to prove that when two chords intersect in a circle, the products of the lengths of their pieces are equal. Inscribed angle theorems for ellipses, hyperbolas and parabolas. Inscribed angle theorems exist for ellipses, hyperbolas and parabolas, too. The essential differences are the measurements of an angle. (An angle  is considered a pair of intersecting lines.)