Read and construct a circle equation by completing squares
Find the centre and radius from either circle form, and build a circle from its centre or diameter.
Pure Mathematics 1 (Paper 1) · AS & A Level Mathematics 9709, 2026–2027. Use algebra and elementary geometry; implicit differentiation is outside this section. Written practice is self-checked.
Before you begin
- Complete a square and compute coordinate distances and midpoints.
- Keep a squared radius distinct from the radius itself.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the centre of (x + 3)² + (y − 4)² = 36?
What you will learn
- Interpret (x − a)² + (y − b)² = r² and its expanded form.
- Use a diameter or completed squares to determine a circle.
A circle fixes the distance from a centre
The equation (x − a)² + (y − b)² = r² describes points at distance r from centre (a, b), for r > 0. The bracket signs come from coordinate differences: (x + 3)² means centre x = −3. The right-hand side is the squared radius, so an equation ending in 16 has radius 4, not 16.
Expanding gives x² + y² − 2ax − 2by + (a² + b² − r²) = 0. In x² + y² + 2gx + 2fy + c = 0, the centre is (−g, −f) and r² = g² + f² − c. Equal non-zero coefficients on x² and y² can first be divided out; arbitrary unequal coefficients do not represent this circle form.
Complete both squares and check the geometry
For x² + y² − 4x + 2y − 20 = 0, write (x − 2)² − 4 + (y + 1)² − 1 − 20 = 0. Thus (x − 2)² + (y + 1)² = 25: centre (2, −1), radius 5. Completing both squares gives a direct check on the memorised coefficient formulas.
A diameter's midpoint is the centre, while half its length is the radius. Equivalently r² is one quarter of the squared endpoint distance. If completing squares produces r² = 0, the real set is one point; if r² < 0, it has no real points. Neither is an ordinary positive-radius circle. Verify a proposed point by substituting into the circle equation.
Pause and explain
What is the radius of (x − 2)² + (y + 1)² = 25?
Worked example
Find the centre and radius of x² + y² − 4x + 2y − 20 = 0.
Show the worked solution
- Complete x² − 4x as (x − 2)² − 4.
- Complete y² + 2y as (y + 1)² − 1, then collect the constants.
- The equation becomes (x − 2)² + (y + 1)² = 25, so read the centre and positive radius.
Answer Centre (2, −1); radius 5.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the circle with centre (−3, 4) and radius 6 in centre-radius and expanded forms.
Give me a hint
Use squared coordinate differences and r² = 36.
Compare my reasoning
- The centre-radius form is (x + 3)² + (y − 4)² = 36.
- Expand to x² + 6x + 9 + y² − 8y + 16 = 36.
- Collect to x² + y² + 6x − 8y − 11 = 0.
(x + 3)² + (y − 4)² = 36; x² + y² + 6x − 8y − 11 = 0.
Look for these in your work
- Use the signed centre correctly.
- Expand and collect every constant.
Find the centre and radius of x² + y² + 6x − 8y + 9 = 0.
Give me a hint
Complete x² + 6x and y² − 8y separately.
Compare my reasoning
- Write (x + 3)² − 9 + (y − 4)² − 16 + 9 = 0.
- Rearrange to (x + 3)² + (y − 4)² = 16.
- The centre is (−3, 4) and the positive radius is 4.
Centre (−3, 4); radius 4.
Look for these in your work
- Compensate correctly for both squares.
- Distinguish the radius from its square.
A circle has diameter endpoints A(−2, 1) and B(4, 5). Find its equation.
Give me a hint
The centre is the midpoint, and r² is AB²/4.
Compare my reasoning
- The midpoint is (1, 3).
- The squared diameter length is 6² + 4² = 52, so r² = 13.
- The circle is (x − 1)² + (y − 3)² = 13; both endpoints are at that squared distance from the centre.
(x − 1)² + (y − 3)² = 13.
Look for these in your work
- Use the diameter midpoint as the centre.
- Divide squared diameter length by 4 to obtain r².
Common mistakes
- Read the centre from the values making both brackets zero.
- Take the positive square root of r² to get r.
- For a diameter, use half its length rather than its full length as the radius.
What can you explain now?
Find the centre and radius of x² + y² − 8x − 6y + 16 = 0.
Compare with the explanation
Centre (4, 3); radius 3.
The completed equation is (x − 4)² + (y − 3)² = 16 + 9 − 16 = 9.
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Make it stick
Derive a circle equation from a distance formula, then reverse the process by completing two squares. Explain the zero and negative squared-radius cases.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.