Equation of a Circle

We will learn how to find the equation of a circle whose centre and radius are given.

Case I: If the centre and radius of a circle be given, we can determine its equation:

To find the equation of the circle whose centre is at the origin O and radius r units:

Equation of a Circle

Let M (x, y) be any point on the circumference of the required circle. 

Therefore, the locus of the moving point M = OM = radius of the circle = r 

⇒ OM$^{2}$ = r$^{2}$

⇒ x$^{2}$ + y$^{2}$ = r$^{2}$, which is the required equation of the circle.

Case II: To find the equation of the circle whose centre is at C (h, k) and radius r units:

Equation of Circle

Let M (x, y) be any point on the circumference of the requited circle. Therefore, the locus of the moving point M = CM = radius of the circle = r

⇒ CM$^{2}$ = r$^{2}$

⇒ (x - h)$^{2}$ + (y - k)$^{2}$ = r$^{2}$, which is the required equation of the circle.

Note: 

(i) The above equation is known as the central from of the equation of a circle.

(ii) Referred to O as pole and OX as initial line of polar co-ordinate system, if the polar co-ordinates of M be (r, θ) then we shall have,

Parametric Equations of a Circle

r = OM = radius of the circle = a and ∠MOX = θ.

Then, from the above figure we get,

x = ON = a cos θ and y = MN = a sin θ

Here, x = a cos θ and y = a sin θ represent the parametric equations of the circle x$^{2}$ + y$^{2}$ = r$^{2}$.

Solved examples to find the equation of a circle:

1. Find the equation of a circle whose centre is (4, 7) and radius 5.

Solution:

The equation of the required circle is

(x - 4)$^{2}$ + (y - 7)$^{2}$ = 5$^{2}$

⇒ x$^{2}$ - 16x + 16 + y$^{2}$ - 14y + 49 = 25

⇒ x$^{2}$ + y$^{2}$ - 16x - 14y + 40 = 0

2. Find the equation of a circle whose radius is 13 and the centre is at the origin.

Solution:

The equation of the required circle is

x$^{2}$ + y$^{2}$ = 13$^{2}$

⇒ x$^{2}$ + y$^{2}$ = 169

● The Circle

• Definition of Circle
• Equation of a Circle
• General Form of the Equation of a Circle
• General Equation of Second Degree Represents a Circle
• Centre of the Circle Coincides with the Origin
• Circle Passes through the Origin
• Circle Touches x-axis
• Circle Touches y-axis
• Circle Touches both x-axis and y-axis
• Centre of the Circle on x-axis
• Centre of the Circle on y-axis
• Circle Passes through the Origin and Centre Lies on x-axis
• Circle Passes through the Origin and Centre Lies on y-axis
• Equation of a Circle when Line Segment Joining Two Given Points is a Diameter
• Equations of Concentric Circles
  
• Circle Passing Through Three Given Points
• Circle Through the Intersection of Two Circles
• Equation of the Common Chord of Two Circles
• Position of a Point with Respect to a Circle
• Intercepts on the Axes made by a Circle
• Circle Formulae
• Problems on Circle 

11 and 12 Grade Math 

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