A circle is a closed shape formed by tracing a point that moves in a plane such that its distance from a given point is constant. The word circle is derived from the Greek word kirkos, meaning hoop or ring. in this article, we cover the important terms related to circles, their properties, and various circle formulas.
Following is a brief outline of the topics that we will cover in this article:
 Definition of a circle
 Important terms related to Circles
 Center
 Radius
 Diameter
 Circumference
 Arc
 Sector
 SemiCircle
 Important Properties of Circles – Related to Lines
 Chord
 Tangent
 Important Properties of Circle – Related to angles and circles
 Inscribed Angle
 Central Angle
 Important Circle Formulas
 Perimeter – Circle Formula
 Area – Circle Formula
Definition of a Circle
When a set of all points that are at a fixed distance from a fixed point are joined then the geometrical figure obtained is called circle.
Let us now learn a bit about the terminology used in circles.
Terms related to Circles
Center
The fixed point in the circle is called the center.
 So, the set of points are at a fixed distance from the center of the circle.
Radius
Radius is the fixed distance between the center and the set of points. It is denoted by “R”.
Diameter
Diameter is a line segment, having boundary points of circles as the endpoints and passing through the center.
 So, logically a diameter can be broken into two parts:
 One part from one boundary point of the circle to the center
 And, the other part from the center to another boundary point.
 Hence, Diameter = Twice the length of the radius or “D = 2R”
Circumference
It is the measure of the outside boundary of the circle.
So, the length of the circle or the perimeter of the circle is called Circumference.
Arc of a circle
The arc of a circle is a portion of the circumference.
From any twopoints that lie on the boundary of the circle, two arcs can be created: A Minor and a Major Arc.
 Minor arc: The shorter arc created by two points.
 Major Arc: The longer arc created by two points.
Sector of a circle:
A Sector is formed by joining the endpoints of an arc with the center.
 On joining the endpoints with the center, two sectors will be obtained: Minor and Major.
 By default, we only consider the Minor sector unless it is mentioned otherwise.
Semicircle
A semicircle is half part of the circle or,
 A semicircle is obtained when a circle is divided into two equal parts.
Now that we know all the terminologies related to the circles, let us learn about the properties of a circle.
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Important Properties of Circle – Lines
Properties related to Lines in a Circle
Chord
A chord is a line segment whose endpoints lie on the boundary of the circle.
Properties of Chord
 Perpendicular dropped from the center divides a chord into two equal parts.
Tangent
Tangent is a line that touches the circle at any point.
Properties of Tangent
 Radius is always perpendicular to the tangent at the point where it touches the circle.
Important Properties of Circle – Related to Angles
Properties related to Angles in a circle
Inscribed Angle
An inscribed angle is the angle formed between two chords when they meet on the boundary of the circle.
Properties of Inscribed Angles
1. Angles formed by the same arc on the circumference of the circle is always equal.
2. The angle in a semicircle is always 90°.Central Angle
A central angle is the angle formed when twoline segments meet such that one of the endpoints of both the line segment is at the center and another is at the boundary of the circle.
Property of Central Angles
 An angle formed by an arc at the center is twice the inscribed angle formed by the same arc.
Important Circle Formulas: Area and Perimeter
The following are some mathematical formulae that will help you calculate the area and perimeter/circumference of a circle.
Perimeter:
 Perimeter or the Circumference of the circle = 2 × π × R.
 Length of an Arc = (Central angle made by the arc/360°) × 2 × π × R.
Area:
 Area of the circle = π × R²
 Area of the sector =(Central angle made by the sector/360°) × π × R².
Summary of all the Properties of a Circle
Here is a summarized list of all the properties we have learned in the article up to this point.
Important Properties  
Lines in a circle  Chord  Perpendicular dropped from the center divides the chord into two equal parts. 
Tangent  Radius is always perpendicular to the tangent at the point where it touches the circle.  
Angles in a circle  Inscribed Angle  1. Angles formed by the same arc on the circumference of the circle is always equal. 2. The angle in a semicircle is always 90. 
Central Angle  The angle formed by an arc at the center is twice the inscribed angle formed by the same arc.  
Important Formulae  Circumference of a circle  2 × π × R. 
Length of an arc 


Area of a circle  π × R²  
Area of a sector 

Application of the properties in the questions
Question 1
The lengths of two sides in a rightangle triangle other than hypotenuse are 6 cm and 8 cm. If this rightangle triangle is inscribed in a circle, then what is the area of the circle?
 5 π
 10 π
 15 π
 20 π
 25 π
Solution
Step 1: Given
 The lengths of two sides other than hypotenuse of a right triangle are 6 cm and 8 cm.
 This triangle is inscribed in a circle.
Step 2: To find
 Area of the circle.
Step 3: Approach and Working out
Let us draw the diagrammatic representation.
By applying the property that the angle in a semicircle is 90º, we can say that AB is the diameter of the circle.
 And, once we find the length of the diameter, we can find the radius, and then we can find the area of the circle as well.
Applying Pythagoras theorem in △ ABC,
 AB² = AC² + BC²
 AB² = 6² + 8²^{ }= 36 +64 = 100
 AB = 10 cm
Since AB is the diameter, AB = 2R = 10
 Hence, R = 5 cm.
Area of the circle = π × R²= π × 5² = 25 π.
Hence, the correct answer is option E.
Question 2
In the diagram given above, O is the center of the circle. If OB = 5 cm and ∠ABC = 30^{0} then what the length of the arc AC?
 5π/6
 5π/3
 5π/2
 5π
 10π
Solution
Step 1: Given
 OB = 5 cm
 ∠ABC = 30°
Step 2: To find
 Length of the arc
Step 3: Approach and Working out
 Length of the arc = (Central angle made by the arc/360°) × 2 × π × R.
To find the length of the arc, we need the value of two variable, the center angle made by the arc and the radius.
 We are already given radius as OB = 5cm
 We need to find the ∠AOC
On visualizing the diagram, the inscribed angle by the arc AC is ∠ABC, and the center angle by arc AC is ∠AOC.
 Hence, we can apply the property that the angle made at the center by an arc is twice the inscribed angle formed by the same arc.
 Thus, ∠AOC = 2 × ∠ABC = 2 × 30°^{ }= 60°
Now, we know the central angle formed by the arc as well.
 Hence, length of the arc AC =(Central angle made by the arc/360°) × 2 × π × R.
 =(60°/360°) × 2 × π × 5.
 =(1/6) × 2 × π × 5.
 =(5π/3) cm
Thus, the correct answer is option B.
If you like this article here are a few more article related to geometry:
 Triangle – Formulas and Concepts (Part1)
 Triangle – Formulas and Concepts (Part2)
 Triangle – Formulas and Concepts (Part3)
 Most common mistakes in Geometry Questions – GMAT Quant
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