circle
Figure 1. To find the equation of a circle, observe that each point on it forms a rightangled triangle whose sides are the x distance and y distance and whose hypotenuse is r. Pythagoras's theorem tells us that x^{ 2} + y ^{2} = r ^{2}. For points outside the circle x ^{2} + y ^{2} exceeds r ^{2}; inside it x ^{2} + y ^{2} is less than r ^{2}. Along the intersection of these two regions the equation balances. All mathematical curves divide space in this way.
Figure 2. Terms associated with a circle.
Figure 3. Inscribed angles of a circle.
A circle is the set of all points in a plane at a given distance r, called the radius, from a fixed point, called the center. A circle is a simple closed curve that divides the plane into an interior and exterior. It has a perimeter, called a circumference, of length 2πr, and encloses an area of πr^{ 2}.
In coordinate geometry a circle with center (x_{0}, y_{0}) and radius r is the set of all points (x, y) such that:
(x  x_{0})^{ 2} + (y  y_{0})^{ 2} = r^{ 2}.
'Circle' comes from the Latin circus, which refers to a large
round or rounded oblong enclosure in which the famous Roman chariot races
were held.
A line cutting a circle in two places is called a secant. The segment of a secant bound by the circle is called a chord, and the longest chord is that which passes through the center and is known as a diameter. The ratio of the circumference to the diameter is π. The length of a circle between two radii is called an arc; the ratio between the length of an arc and the radius defines the angle between two radii in radians. The area bounded by two radii and an arc is known as a sector.
A segment is part of a circle bounded by a chord and the arc subtending the chord. A line touching a circle in one place is called a tangent. Tangent lines are perpendicular to radii.
In affine geometry all circles and ellipses become congruent, and in projective geometry the other conic sections join them. A circle is a conic section with eccentricity zero. In topology all simple closed curves are homeomorphic to circles, and the word circle is often applied to them as a result. The threedimensional analog of the circle is the sphere, and the fourdimensional analog is the hypersphere.
Butterfly theorem
The butterfly theorem may be stated as follows: Let M be the midpoint of a chord PQ of a circle, through which two other chords AB and CD are drawn. If AD intersects PQ at X and CB intersects PQ at Y, then M is also the midpoint of XY. The theorem gets its name from the shape of the resulting figure.
Inscribed angle theorems
Referring to Figure 3, angle O = 2B = 2E. The general theorem that follows from this is that an inscribed is half the central angle. Also, B = E. In general, angles subtended by the same arc are equal.
Note, by the way, that the circumference of the arc ADC is πrO/180.
Johnson's theorem
If three congruent circles all intersect in a single point, then the other three points of intersection will lie on another circle of the same radius. This simple little theorem was discovered by Roger Johnson in 1916.
Ninepoint circle
Draw a triangle, any triangle (although it may be best to start with an acute triangle). Mark the midpoints of each side. Drop an altitude from each vertex to the opposite side, and mark the points where the altitudes intersect the opposite side. (If the triangle is obtuse, an altitude will be outside the triangle, so extend the opposite side until it intersects.) Notice that the altitudes intersect at a common point. Mark the midpoint between each vertex and this common point. No matter what triangle you start with, these nine points all lie on a perfect circle! This result was known to Leonhard Euler in 1765, but was rediscovered by the German mathematician Karl Feuerbach (1800–1834) in 1822.
Inscribed circle
An inscribed circle is a circle inside a triangle or other polygon whose edges are tangents to the circle. Such a polygon is said to be circumscribed (see circumscription) to the circle. The (unique) circle inscribed to a triangle is called the incircle.
Circle involute
A circle involute is the simplest kind of spiral to draw and understand. It is, for example, the path that a goat, tethered to a post, would follow if it walked around and around in the same direction, keeping its tether taut until it wound its way to the center.
Figure 4. Circle involute

The radial distance between adjacent loops of the spiral is equal to the circumference of the central circle. Except for the innermost loop, the circle involute is hard to distinguish from the Archimedean spiral, though the two curves are never identical.