Properties of Circles – From Intuition to Precision
A circle is the set of all points that lie at a fixed distance from a single point. That single point is the centre, and the fixed distance is the radius. If you tie a string to a pencil, hold the other end fixed, and swing the pencil around, the curve it traces is a circle. Every point on that curve is exactly one string-length away from your finger.
That simple idea — constant distance from a centre — is the seed from which every property of a circle grows.
The Basic Parts
Before we talk about properties, name the pieces.
- Centre (O): the fixed point.
- Radius (r): the distance from centre to any point on the circle.
- Diameter (d): a line segment through the centre, joining two points on the circle. d=2r.
- Chord: any line segment joining two points on the circle. The diameter is the longest chord.
- Arc: a part of the circumference (the curved boundary).
- Sector: the region between two radii and the arc they cut off.
- Segment: the region between a chord and the arc it cuts off.
The circumference is the perimeter of the circle: C=2πr.
The area enclosed is A=πr2.
π (pi) is the constant ratio of circumference to diameter — about 3.14159.
Property 1: Equal Chords Are Equidistant from the Centre
Take a circle. Draw two chords of equal length. Drop perpendiculars from the centre to each chord. Those perpendicular distances will be equal. Conversely, if two chords are at the same distance from the centre, they are equal in length.
Why? The perpendicular from the centre to a chord bisects the chord. So each chord is split into two equal halves. If the chords are equal, the right triangles formed by the radius, the half-chord, and the perpendicular are congruent — hence the perpendicular distances match.
To find the length of a chord given its distance from the centre, use the right triangle:
(half-chord)2+(distance)2=r2.
Property 2: The Angle Subtended by a Chord at the Centre Is Twice the Angle Subtended at Any Point on the Circumference
This is the most powerful property of circles. Draw a chord AB. Let O be the centre. Pick any point P on the circumference (on the same side of AB). Then:
∠AOB=2×∠APB
The central angle is always double the inscribed angle.
Why? Join OA, OB, OP. You get two isosceles triangles. The exterior angle of a triangle equals the sum of the two opposite interior angles. Work through the geometry — the doubling falls out naturally.
A special case: if AB is a diameter, then ∠AOB=180∘, so ∠APB=90∘.
The angle in a semicircle is a right angle. This is a classic exam favourite.
Property 3: Angles in the Same Segment Are Equal
If you take a chord AB and pick two different points P and Q on the same arc (the same side of AB), then:
∠APB=∠AQB
Both are half of the same central angle ∠AOB, so they must be equal. This is a direct consequence of Property 2.
Property 4: The Tangent Is Perpendicular to the Radius at the Point of Contact
A tangent is a line that touches the circle at exactly one point. At that point, the radius drawn to the point is perpendicular to the tangent.
Why? Suppose the radius were not perpendicular. Then you could draw a shorter line from the centre to the tangent, meaning the tangent would cut the circle at two points — contradiction. So the radius must be the shortest distance from centre to tangent, which forces a right angle.
A common mistake: thinking a tangent is perpendicular to the chord at the point of contact. No — it is perpendicular to the radius, not the chord.
Property 5: The Lengths of Two Tangents from an External Point Are Equal
From a point outside the circle, you can draw exactly two tangents. Their lengths (from the external point to the points of contact) are equal.
Proof: Join the external point P to the centre O, and draw radii to the two points of contact A and B. Triangles POA and POB are congruent (right angle, common hypotenuse, equal radii). Hence PA=PB.
Putting It All Together
These five properties are the backbone of every circle problem you will face in exams. They connect chords, angles, arcs, and tangents into a single logical structure. When you see a circle question, ask yourself:
- Is there a chord? Drop a perpendicular from the centre.
- Is there an angle at the circumference? Look for the central angle.
- Is there a tangent? Draw the radius to the point of contact.
- Is there an external point? Check for equal tangent lengths.
The geometry of the circle is not a list of facts — it is a web of relationships, all flowing from the simple idea of constant distance from a centre.