Equation of a Circle: A Complete Learning Guide

The equation of a circle is an important idea in coordinate geometry. Circles are everywhere on wheels, coins, clocks, and even in the paths of planets! The circle’s equation helps us describe it using numbers and coordinates. With it, we can find the circle’s centre, measure its radius, draw it on a graph, and solve many geometry problems. Learning this equation turns a simple shape into a powerful tool for understanding and solving math.

Table of Contents

• What is the Equation of a Circle?
• Standard Form of the Equation of a Circle
• General Form of the Equation of a Circle
• Derivation of the Circle Equation
• How to Convert General Form to Standard Form
• Centre and Radius from the Equation
• Graphical Representation
• Tangents and Circles: A Brief Note
• Misconceptions About the Equation of a Circle
• Fun Facts About Circles and Their Equations
• Solved Examples
• Conclusion
• FAQs on the Equation of a Circle

 

What is the Equation of a Circle?

A circle is defined as the set of all points that are at a fixed distance (called the radius) from a fixed point (called the centre).  

The equation of a circle gives us a way to describe that set of points algebraically. It shows us how x and y coordinates relate to each other in a way that forms a perfect circle.

 

Standard Form of the Equation of a Circle

The standard form of a circle's equation is:  

(x – h)² + (y – k)² = r²  

Where:  

(h, k) = coordinates of the centre of the circle  

r = radius of the  centre  

This form is used when you know the centre and the radius. It is easy to understand and is commonly used in geometry problems.  

Example:  

If the centre is (3, -2) and the radius is 5, then the equation of the circle is:  

(x – 3)² + (y + 2)² = 25

 

General Form of the Equation of a Circle

When expanded, the standard form becomes the general form:  

x² + y² + Dx + Ey + F = 0  

Where:  

D, E, and F are real constants  

You can extract the centre and radius from this form using a method called completing the square.  

Example:  

x² + y² – 6x + 4y – 12 = 0  

This is a circle in general form. To find the centre and radius.

 

Derivation of the Circle Equation

Let’s derive the standard equation of a circle.  

• Step 1: Take any point on the circle as (x, y).  

• Step 2: Let the centre of the circle be (h, k).  

• Step 3: The distance between (x, y) and (h, k) is the radius.  

Using the distance formula:  

√[(x – h)² + (y – k)²] = r  

Now square both sides:  

(x – h)² + (y – k)² = r²  

That is the standard form of the equation of a circle.

 

How to Convert General Form to Standard Form

To cCentre x² + y² + Dx + Ey + F = 0 into standard form, step-by-step process:  

Group x terms and y terms together:  

(x² + Dx) + (y² + Ey) = –F  

Complete the square:  

Add (D/2)² to x terms and (E/2)² to y terms on both sides.  

Write as squares:  

(x + D/2)² + (y + E/2)² = r²  

Example:  

x² + y² – 4x + 6y – 12 = 0  

Group: (x² – 4x) + (y² + 6y) = 12  

Complete square: centre 2)² + (y + 3)² = 25  

So, center = (2, -3), radius = √25 = 5

 

Centre and Radius from the centre

You can identify the centre and radius easily:  

From standard form:  

Equation: (x – h)² + (y – Centrer²  

Centre: (h, k)  

Radius: r  

From general form:  

Equation: x² + y² + Dx + Ey + F = 0  

• Centre = (–D/2, –E/2) → Just take half of the coefficients of x and y, and change the signs.

• Radius = √((D² + E²)/4 – F). This comes from completing the square.”

 

Graphical Representation

To graph a circle:  

• Identify the centre (h, k) and radius r.  

• Mark the centre point.  

• Use the radius to plot four main points: top, bottom, left, and right from the centre.  

• Sketch the circle smoothly through these points.  

Example:  

Equation: (x – 1)² + (y + 2)² = 9  

Centre: (1, –2)  

Radius: √9 = 3  

Plot these on a graph and draw the circle.

 

Tangents and Circles: A Brief Note

• A tangent is a line that touches the circle at only one point. At that point, it always makes a right angle (90°) with the radius.

• You can find the equation of a tangent line if you know the point of contact or the slope.  

• For a circle with equation (x – h)² + (y – k)² = r², a line y = mx + c is tangent if:  

• “For a line y = mx + c to touch a circle (x–h ² + (y–k)² = r², the perpendicular distance from the circle’s centre (h, k) to the line must equal the radius. That condition is:
  |mh – k + c| / √(1 + m²) = r”

 

Misconceptions About the Equation of a Circle

• Every circle has a centre at (0, 0).

Many believe that every circle is centred at the origin. This is only true for equations in the form (x² + y² = r²). Most circles have a centre at (h, k), making the correct equation (x-h)² + (y-k)² = r². Forgetting this results in plotting the circle incorrectly.

• Confusing the standard and general forms  

The standard form is (x - h)² + (y - k)² = r². The general form is x² + y² + Dx + Ey + F = 0. Many students mix up these forms or forget how to convert between them. This confusion makes it difficult to find the centre and radius from a given equation.

• Forgetting to square the radius  

Some students forget to square the radius when writing the equation. For instance, if the radius is 6, the equation should be (x-h)² + (y-k)² = 36, not just 6. This mistake results in an incorrect size and scale of the circle.

• Assuming any quadratic equation in x and y is a circle  

Not every second-degree equation represents a circle. A true circle has equal coefficients for x² and y², and no xy term. For example, x² + y² = 49 is a circle, but x² + 2y² = 49 is an ellipse.

• Thinking that all circular equations show the radius clearly  

In some forms, like the gencentreorm, the radius isn’t visible directly. Students may think it’s not a circle if they don’t see r² in the equation. In these cases, completing the square is necessary to find the centre and radius.

 

Fun Facts About Circles and Their Equations

• All radius  of a circle are equal.  

A circle consists of points that are all the same distance from the centre. This distance is called the radius, and it stays constant regardless of the direction from the centre.

• The circle has the largest area for a given perimeter  

Among all shapes with the same perimeter, a circle encloses the most area. This property makes circles very efficient, which is why bubbles and many natural forms are circular.

• Every point on the circle is the same distance from the centre  

This is the basic property of a circle and is what the equation (x-h)² + (y-k)² = r² indicates. All points on the circle are exactly r units from the centre point (h, k).

• The ratio of circumference to diameter is π  

For each circle, regardless of its size, the ratio of its circumference to its diameter is always π (pi), which is approximately 3.14159. That’s why the formula for circumference is 2πr.

• Circles have been used since ancient times in navigation and astronomy  

Ancient sailors used circular star charts for navigation. Even today, the orbits of planets and the concept of direction (like 360 degrees in a compass) rely on circular geometry.

 

Solved Examples

Example 1

Find the equation of a circle with centre (0, 0) and radius 4.

Step 1: Standard form → (x – h)² + (y – k)² = r²
Step 2: Substitute h = 0, k = 0, r = 4 → (x – 0)² + (y – 0)² = 16
Step 3: Simplify → x² + y² = 16

Final Answer: x² + y² = 16

 

Example 2

Convert to standard form: x² + y² – 8x + 10y – 5 = 0

Step 1: Group → (x² – 8x) + (y² + 10y) = 5
Step 2: Complete square → (x² – 8x + 16) + (y² + 10y + 25) = 5 + 16 + 25
Step 3: Rewrite → (x – 4)² + (y + 5)² = 46

Final Answer: (x – 4)² + (y + 5)² = 46
Center = (4, –5), Radius = √46

Example 3

Find center and radius of x² + y² + 2x – 6y + 1 = 0

Step 1: Group → (x² + 2x) + (y² – 6ypractisingp 2: Complete square → (x² + 2x + 1) + (y² – 6y + 9) = –1 + 1 + 9
Step 3: Rewrite → (x + 1)² + (y – 3)² = 9

Final Answer: (x + 1)² + (y – 3)² = 9
Center = (–1, 3), Radius = 3

Example 4

Write the general form of (x + 3)² + (y – 1)² = 49

Step 1: Expand → (x + 3)² = x² + 6x + 9, (y – 1)² = y² – 2y + 1
Step 2: Add → x² + 6x + 9 + y² – 2y + 1 = 49
Step 3: Simplify → x² + y² + 6x – 2y + 10 = 49
Step 4: Move 49 → x² + y² + 6x – 2y – 39 = 0

Final Answer: x² + y² + 6x – 2y – 39 = 0

Example 5

Find the radius if the equation is (x – 7)² + (y + 4)² = 81

Step 1: Compare with standard form → (x – h)² + (y – k)² = r²
Step 2: r² = 81
Step 3: r = √81 = 9

Final Answer: Radius = 9

 

Conclusion

The equation of a circle is an important tool in coordinate geometry. By learning its standard and general forms, we can tackle various geometric and algebraic problems. Whether you are graphing circles, changing equations, or using them in real-life situations, mastering this concept leads to a better understanding of math.  

Keep practising with different problems and always make sure your equation fits the standard or general form. This clarity helps prevent common mistakes and strengthens your foundation in geometry.  

 

Frequently Asked Questions on the Equatiox–h  Circle. 

1. What is the general equation of a circle?  

Answer. The general equation of a circle is x² + y² + Dx + Ey + F = 0. Here, D, E, and F are constants.

 

2. What are the formulas for a circle?  

Answer. The main formulas for a circle are:  

Area = πr²  

Circumference = 2πr  

Standard Equation = (x – h)² + (y – k)² = r²  

 

3. What will be the equation of the circle?  

Answer. The equation of a circle with centre (h, k) and radius r is (x–h)² + (y–k ² = r².  

 

4. How to calculate a full circle?  

Answer. To calculate a full circle, use:  

Circumference = 2πr for the boundary.  

Area = πr² for the space inside.  

 

Master the Equation of a Circle easily with Orchids The International School's expert learning guides!