3.2.8

Equation of a Circumcircle

Test yourself

Equation of a Circumcircle of a Triangle from Three Points

We can find the general equation of a circumcircle of a triangle by using the properties of chords.

Illustrative background for What is a circumcircle?Illustrative background for What is a circumcircle? ?? "content

What is a circumcircle?

  • The circumcircle of a triangle is the circumference of a circle upon which the three vertices of the triangle lie.
    • A group of three points can only have one circumcircle.
Illustrative background for What is a circumcircle?Illustrative background for What is a circumcircle? ?? "content

What is a circumcircle?

  • Each side of the triangle is a chord of the circle.
  • The perpendicular bisectors of the triangle sides intersect at the centre of the circle.
Illustrative background for How do you find the equation of a circumcircle?Illustrative background for How do you find the equation of a circumcircle? ?? "content

How do you find the equation of a circumcircle?

  • To write the general equation of the circumcircle, we need to find the centre and the radius of the circle.
Illustrative background for How do you find the equation of a circumcircle?Illustrative background for How do you find the equation of a circumcircle? ?? "content

How do you find the equation of a circumcircle?

  • The centre is found by working out the point of intersection between the perpendicular bisectors of two of the sides of the triangle.
  • The radius is found by working out the distance between the centre of the circle and one of the points of the triangle.
Illustrative background for ExampleIllustrative background for Example ?? "content

Example

  • The points A(-6,3), B(-3,2) and C(0,3) lie on the circumference of a circle.
    • What is the equation of the circle?
Illustrative background for Find the perpendicular bisector of ABIllustrative background for Find the perpendicular bisector of AB ?? "content

Find the perpendicular bisector of AB

  • The gradient of the chord AB is equal to:
    • 2−3(−3)−(−6)=−13 \frac{2-3}{(-3) - (-6)}=-\frac{1}{3}
  • The gradient of the perpendicular bisector of AB must be equal to 3.
  • The midpoint of AB has coordinates:
    • Midpoint = ((−6)+(−3)2,3+22)=(−4.5,2.5)(\frac{(-6)+(-3)}{2},\frac{3+2}{2}) = (-4.5,2.5)
Illustrative background for Find the perpendicular bisector of ABIllustrative background for Find the perpendicular bisector of AB ?? "content

Find the perpendicular bisector of AB

  • The intercept of the perpendicular bisector, cc is then found from:
    • y=3x+cy = 3x + c
Illustrative background for Find the perpendicular bisector of ABIllustrative background for Find the perpendicular bisector of AB ?? "content

Find the perpendicular bisector of AB

  • Substituting in the midpoint coordinates, we get:
    • 2.5=(3×(−4.5))+c2.5 = (3 \times (-4.5)) + c
    • 2.5=−13.5+c→c=162.5 = -13.5 +c \rightarrow c = 16
  • So the equation of the perpendicular bisector of AB is y=3x+16y = 3x +16
Illustrative background for Find the perpendicular bisector of ACIllustrative background for Find the perpendicular bisector of AC ?? "content

Find the perpendicular bisector of AC

  • The gradient of the chord AC is equal to:
    • 3−30−(−6)=0 \frac{3-3}{0 - (-6)}=0
  • This means that AC is horizontal.
  • The equation for the perpendicular bisector must be:
    • x=(−6)+02=−3x = \frac{(-6)+0}{2} = -3
Illustrative background for Find the centre of the circumcircleIllustrative background for Find the centre of the circumcircle ?? "content

Find the centre of the circumcircle

  • The centre of the circumcircle is the point of intersection between the perpendicular bisectors of AB and AC.
Illustrative background for Find the centre of the circumcircleIllustrative background for Find the centre of the circumcircle ?? "content

Find the centre of the circumcircle

  • Substituting in x=3x=3 into y=3x+16y = 3x +16, we get:
    • y=(3×3)+16→y=7y = (3 \times 3) +16 \rightarrow y = 7
  • The centre of the circumcircle has coordinates (3,7).
Illustrative background for Find the radius of the circumcircleIllustrative background for Find the radius of the circumcircle ?? "content

Find the radius of the circumcircle

  • The radius of the circumcircle is equal to the distance between the centre and any one of the points of the triangle.
Illustrative background for Find the radius of the circumcircleIllustrative background for Find the radius of the circumcircle ?? "content

Find the radius of the circumcircle

  • The distance between the centre and point AA is found by using Pythagoras' theorem:
    • Distance =(3−(−6))2+(7−3)2=5= \sqrt{(3-(-6))^2 + (7-3)^2} = 5
  • The radius of the circumcircle is equal to 5.
Illustrative background for Write the equation of the circumcircleIllustrative background for Write the equation of the circumcircle ?? "content

Write the equation of the circumcircle

  • We can now write the equation of the circumcircle in its general form:
    • (x+3)2+(y−7)2=25(x+3)^2 + (y-7)^2 = 25

Jump to other topics

1Proof

1.1Types of Numbers

1.1.1Whole Numbers

1.1.2The Four Rules for Positive Numbers

1.1.3Negative Numbers

1.1.4Integers

1.1.5The Four Rules for Negative Numbers

1.1.6Real Numbers

1.1.7Rational vs Irrational

1.2Notation

1.2.1Sets

1.2.2Subsets

1.2.3Venn Diagrams

1.2.4Sets Numbers

1.2.5Notation in Mathematical Arguments

1.3Proof

1.3.1Mathematical Proof

1.3.2Proof by Deduction

1.3.3Proving an Identity

1.3.4Polynomial Identities

1.3.5Proof by Exhaustion

1.3.6Disproof by Counter Example

1.3.7Proof by Contradiction (A2 Only)

2Algebra & Functions

2.1Powers & Roots

2.1.1Exponential Rules

2.1.2Rational Exponents

2.1.3Surds

2.1.4Simplify Surds

2.1.5Rationalise Denominator

2.2Quadratic Equations

2.2.1Quadratic +/-

2.2.2Factorise Quadratics

2.2.3Quadratic Formula

2.2.4Complete the Square

2.2.5Solve Quadratics Graphically

2.2.6Simultaneous Equations

2.2.7Simultaneous Quadratic Equations

2.2.8Simultaneous Equations Graphs

2.2.9Simultaneous Equations Word Problems

2.2.10Quadratic Equations of Powers

2.3Inequalities

2.3.1Solving Inequalities

2.3.2Solving Double Inequalities

2.3.3Solving Quadratic Inequalities

2.3.4Representing Inequalities

2.3.5Graphing Inequalities

2.3.6Set Notation

2.3.7Set Notation & Inequalities

2.4Polynomials

2.4.1Polynomials

2.4.2Adding & Subtracting Polynomials

2.4.3Remainder Theorem

2.4.4Algebraic Long Division

2.4.5Rewriting Rational Expressions

2.5Graphs

2.5.1Graphs of Polynomials

2.5.2Simultaneous Equations Graphs

2.5.3Simultaneous Quadratic Equations

2.5.4Graphs of Proportion

2.5.5Graphs of Common Functions 1

2.5.6Graphs of Common Functions 2

2.5.7Graphs of Common Functions 3

2.5.8Proportion

2.5.9Direct Proportion

2.5.10Inverse Proportion

2.6Functions

2.6.1Properties of Functions

2.6.2Inverse Functions

2.6.3Composite Functions

2.7Transformation of Graphs

2.7.1Reflections

2.7.2Rotation

2.7.3Translation

2.7.4Stretches & Compressions

2.7.5Effects of Transformations

2.7.6Combinations of Transformations

2.8Partial Fractions (A2 Only)

2.8.1Partial Fractions

2.8.2Partial Fractions with Three Linear Factors

2.8.3Partial Fractions with Three Linear Factors 2

2.8.4Partial Fractions with Repeated Factors

2.8.5Partial Fractions - Practice

3Coordinate Geometry

3.1Straight Lines

3.1.1y = mx +c

3.1.2Gradient & Y-intercept

3.1.3Distance-Time Graph

3.1.4Velocity-Time Graph

3.1.5Conversion Graph

3.2Circles

3.2.1Equations of Circles centred at Origin

3.2.2Finding the Centre & Radius

3.2.3Equation of a Tangent

3.2.4Circle Theorems - Perpendicular Bisector

3.2.5Circle Theorems - Angle at the Centre

3.2.6Circle Theorems - Angle at a Semi-Circle

3.2.7Equation of a Perpendicular Bisector

3.2.8Equation of a Circumcircle

3.2.9Circumcircle of a Right-angled Triangle

3.3Parametric Equations (A2 only)

3.3.1Parametric Equations

3.3.2Modelling with Parametric Equations

4Sequences & Series

4.1Binomial Expansion

4.1.1Pascal's Triangle

4.1.2Factorials

4.1.3Binomial Expansion

4.1.4Binomial Expansion & Rational Powers (A2 only)

4.1.5Binomial Expansion & Rational Powers 2 (A2 only)

4.2Sequences (A2 only)

4.2.1Nth term Rules

4.2.2Recurring Rules

4.2.3Types of Sequences

4.2.4Arithmetic Sequences

4.2.5Geometric Sequences

4.2.6Sigma Notation

4.2.7Arithmetic Series

4.2.8Geometric Series

4.2.9Sum to Infinity of a Geometric Series

4.2.10Modelling with Sequences & Series

5Trigonometry

5.1Using Angles

5.1.1Radians

5.1.2Arc length

5.1.3Area of Sectors

5.1.4Area of Segments

5.2Trigonometric Functions

5.2.1Unit Circle

5.2.2Properties of Trig Functions

5.2.3Trigonometry

5.2.4Exact values

5.2.5Graphs of Trigonometric Functions

5.2.6Small Angle Approximation

5.3Triangle Rules

5.3.1Sine Rules

5.3.2Cosine Rules

5.3.3Area of a Triangle

5.4Trigonometric Identities

5.4.1Secant, Cosecant & Cotangent

5.4.2Pythagorean Identities

5.4.3Angle Addition Formulae

5.4.4Manipulating the Angle Addition Formulae

5.4.5Double Angle Formula

6Exponentials & Logarithms

6.1Exponentials & Logarithms

6.1.1Exponential Function

6.1.2Logarithms

6.1.3Properties of Logarithms

6.1.4Natural Logarithm

6.1.5Graphs of Exponents & Logarithms

6.2Exponential Growth Decay

6.2.1Population Growth

6.2.2Depreciation

6.2.3Graphing Linear and Exponential Growth

6.2.4Interpreting Exponential Growth and Decay Graphs

6.2.5Compound Growth & Decay

6.2.6Compund Interest

7Differentiation

7.1Derivatives

7.1.1Gradient of a Curve

7.1.2Differentiation from First Principles

7.1.3Differentiation from First Principles - Practice

7.1.4Differentiation

7.1.5Finding derivatives

7.1.6Second Order Derivatives

7.2Graphs & Differentiation

7.2.1Stationary Points

7.2.2Maxima & Minima

7.2.3Modelling with Differentiation

7.2.4Using Second Derivatives (A2 only)

7.2.5Rate of Change (A2 only)

7.3Differentiation With Trigonometry and Exponentials

7.3.1Differentiation with Sin & Cos

7.3.2Diff. of Exponential Functions

7.3.3Differentiation of Trig. Functions 1 (A2 only)

7.3.4Differentiation of Trig. Functions 2 (A2 only)

7.4Rules of Differetiation (A2 only)

7.4.1Chain Rule

7.4.2Product Rule

7.4.3Quotient Rule

7.5Parametric & Implicit Differentiation

7.5.1Parametric Differentiation

7.5.2Implicit Differentiation

8Integration

8.1Integration

8.1.1Integration - Basics

8.1.2Integration - Definite Integrals

8.1.3Finding Functions

8.1.4Area under curves

8.1.5Area under x-axis

8.1.6Area between curves and lines

8.2Integration (A2 Only)

8.2.1Integration - Standard Functions

8.2.2Integration - Reverse Chain Rule

8.2.3Integration by Substitution

8.2.4Integration by Parts

8.2.5Integration by Parts - Practice

8.2.6Integration by Partial Fraction

8.2.7Modelling with Differential Equations

9Numerical Methods

9.1Finding Solutions

9.1.1Number Iterations

9.1.2Iterations to 1 Decimal Places

9.1.3Iteration Formula

9.1.4Locating Roots

9.1.5Newton Raphson Method

9.2Finding the Area

9.2.1Trapezium Rule

10Vectors

10.12D Vectors

10.1.1Vectors - Magnitude & Direction

10.1.2Unit Vectors

10.1.3Vectors - Algebraic Operations

10.1.4Position Vectors

10.1.5Vector Proofs

10.1.6Vectors Word Questions

10.23D Vectors

10.2.13D Vector Magnitude

10.2.23D Vectors - Algebraic Operations

10.2.3Position Vectors 3D

10.2.43D Vectors Word Questions

10.3Vector Proofs

10.3.1Coming Soon

Go student ad image

Unlock your full potential with GoStudent tutoring

  • Affordable 1:1 tutoring from the comfort of your home

  • Tutors are matched to your specific learning needs

  • 30+ school subjects covered

Book a free trial lesson

Equation of a Perpendicular Bisector

Circumcircle of a Right-angled Triangle