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Equation of a Circle (Cambridge O Level Additional Maths)
Revision Note
Equation of a Circle
What is the equation of a circle?
- A circle with centre (a, b) and radius r has the equation
- You need to be able to find the equation of a circle given its centre and radius
- Substitute the values into the formula
How do I find the centre and radius of a centre given its equation?
- Make sure it is in the form
- The radius is the positive square root of the constant term
- The coordinates of the centre can be found by finding the values that make each bracket equal to zero
Examiner Tip
- Remember that the numbers in the brackets have the opposite signs to the coordinates of the centre
- Don't forget to take the square root of the right-hand side of the equation when finding the radius
Worked example
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Finding the Centre & Radius
What are the different forms of the equation of a circle?
- The most useful equation of a circle is
- This is so the centre, and radius are easy to see
- Any other form of the equation of a circle can be rearranged into this form
- The most common alternative form for the equation of a circle is called the general form
- The most common alternative form for the equation of a circle is called the general form
How do I find the centre and radius of a circle from any form of its equation?
- A circle equation in a different form can always be rearranged into (x- a)2 + (y - b)2 = r2
- The centre is then and radius
- Rearranging to this form will often involve completing the square
Worked example
Intersection of a Circle & a Line
What is meant by the intersection of a circle and a line?
- A line may pass through a circle
- in which case it will intersect the circle twice
- the part of the line between the two points of intersection will be a chord
- or, if it passes through the centre of the circle, a diameter
- A line may touch a circle
- in which case it will intersect the circle once
- such a line would be called a tangent to the circle
- A line may not intersect a circle at all
How do I determine whether a line and a circle intersect?
- For the equation of a circle in the form and the equation of a line in the form
- STEP 1
Substitute the linear equation into the circle equation- e.g.
and
would become
- e.g.
- STEP 2
Expand, rearrange and simplify this equation - it should be a quadratic- e.g.
- e.g.
- STEP 3
Solve the equation to deduce the number of intersections
If there are two solutions, there are two intersections, one solution (repeated) indicates a tangent, no (real) solutions indicates no intersection- e.g.
Two solutions so the line and the circle intersect twice
- e.g.
- STEP 4
If required, find the -coordinates of the intersection(s)- e.g.
The line and the circle intersect at the points and
- e.g.
Examiner Tip
- A horizontal or vertical line could intersect a circle, when the full method shown above is unnecessary
- Horizontal lines have the form , so substitute for in the circle equation (and solve for )
- Vertical lines have the form , so substitute for in the circle equation (and solve for )
Worked example
Show that the line is tangent to the circle .
State the coordinates of the point of intersection between the tangent and the circle.
STEP 1 - Substitute the linear equation into the circle equation
STEP 2 - Rearrange to a quadratic
STEP 3 - Solve
(repeated)
There is only one point of intersection so the line is tangent to the circle
STEP 4 - The coordinates are required
The line and circle intersect at
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