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A Level · Cambridge (CIE) · 9709

Find a circle tangent from its perpendicular radius

Check the point lies on the circle, then use the radius direction to construct a tangent without calculus.

Pure Mathematics 1 · Coordinate geometry · 1.3

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

  • Find a circle's centre and check a point in its equation.
  • Build perpendicular lines from points and gradients.

Work on paper. Try each question before revealing help, and explain why your method works.

Quick readiness check

Which radius gradient connects C(2, −1) to P(5, 3)?

What you will learn

  • Use a tangent perpendicular to the radius at the point of contact.
  • Handle horizontal and vertical tangents and intersections of two tangents.
Radius perpendicular to tangent-4-202468-6-4-2024xyC(2, −1)P(5, 3)Radius 5Tangent: 3x + 4y = 27
The circle (x − 2)² + (y + 1)² = 25 has centre C(2, −1) and radius 5. The radius to P(5, 3) changes x by 3 and y by 4. Its gradient 4/3 is perpendicular to tangent gradient −3/4, giving 3x + 4y = 27. The small square marks the right angle at P. Both axes use equal unit lengths; the tangent is shown only within the displayed window.

Begin with the centre and contact point

A circle's tangent at P is perpendicular to the radius CP and passes through P. First verify that P lies on the circle; a line through an arbitrary point with a perpendicular direction is not automatically a tangent there. For centre C(2, −1) and P(5, 3), the squared radius distance is 3² + 4² = 25, so P belongs to the circle (x − 2)² + (y + 1)² = 25.

The radius gradient is (3 + 1)/(5 − 2) = 4/3, hence the tangent gradient is −3/4. Through P, its equation is y − 3 = −3(x − 5)/4, or 3x + 4y = 27. This uses the radius-tangent theorem and line equations; implicit differentiation is not part of this section's method.

Use geometry for exceptional directions

If a radius is vertical, the tangent is horizontal. If a radius is horizontal, the tangent is vertical. For a circle with centre (−1, 2) and radius 5, its top point (−1, 7) has tangent y = 7. A reciprocal-gradient calculation is unnecessary and undefined for a horizontal radius, so handle the geometry directly.

Two tangents from the same exterior point to a circle have equal lengths. One way to check is to use the two right triangles formed by the common centre and the contact points: they have the same centre-to-exterior distance and equal radii. If the contact points are supplied, construct both tangent equations, solve them simultaneously and then calculate the distances to verify the result.

Pause and explain

Which line is the tangent to (x − 2)² + (y + 1)² = 25 at (5, 3)?

Put the idea to work

Worked example

Find the tangent to (x − 2)² + (y + 1)² = 25 at P(5, 3).

Show the worked solution
  1. The centre is C(2, −1), and CP² = 3² + 4² = 25, confirming P lies on the circle.
  2. The radius gradient is 4/3; its perpendicular tangent gradient is −3/4.
  3. Use y − 3 = −3(x − 5)/4 and rearrange to 3x + 4y = 27.

Answer 3x + 4y = 27.

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Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.

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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.

1 · Guided

Find the tangent to x² + y² = 25 at (3, 4).

Give me a hint

The centre is the origin; find the radius gradient and its negative reciprocal.

Compare my reasoning
  1. The point satisfies 3² + 4² = 25, and the radius gradient is 4/3.
  2. The tangent has gradient −3/4 through (3, 4).
  3. Write y − 4 = −3(x − 3)/4 and simplify to 3x + 4y = 25.

3x + 4y = 25.

Look for these in your work

  • Verify the contact point lies on the circle.
  • Use a perpendicular gradient and the correct point.
2 · Independent

A circle has centre (−1, 2) and radius 5. Find its tangent at (−1, 7).

Give me a hint

The contact point and centre have the same x-coordinate.

Compare my reasoning
  1. The point is 5 units above the centre, so it lies on the circle.
  2. The radius is vertical, making the tangent horizontal.
  3. The horizontal line through the contact point is y = 7.

y = 7.

Look for these in your work

  • Recognise the exceptional vertical-radius case.
  • Position the horizontal tangent through the contact point.
3 · Transfer

Tangents to x² + y² = 25 at (3, 4) and (−3, 4) meet at T. Find T and both tangent lengths.

Give me a hint

Build both tangent equations and solve them together.

Compare my reasoning
  1. The equations are 3x + 4y = 25 and −3x + 4y = 25.
  2. Subtract to get x = 0, then y = 25/4, giving T = (0, 25/4).
  3. Each tangent length is √(3² + (25/4 − 4)²) = √(9 + 81/16) = 15/4; the equal lengths also follow from the tangent theorem.

T = (0, 25/4); both tangent lengths are 15/4.

Look for these in your work

  • Construct a valid equation for each tangent.
  • Find the intersection and verify equal lengths using distance.

Common mistakes

  • The perpendicular radius is drawn to the contact point, not to an arbitrary point on the line.
  • Check that the given contact point is on the circle.
  • Use horizontal or vertical line equations in the exceptional gradient cases.
Recall without your notes

What can you explain now?

For the circle x² + y² = 25, find the tangent at its rightmost point (5, 0).

Compare with the explanation

x = 5.

The radius from (0, 0) to (5, 0) is horizontal, so its perpendicular tangent is the vertical line through (5, 0).

After trying it yourself, choose your next review. This is your self-assessment.

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Make it stick

Explain why a tangent must pass through the contact point and be perpendicular to its radius. Construct examples with both finite and undefined radius gradients.

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.

Keep learning Mathematics

Course scope and assessment

312 sequenced lessons provide 936 guided, independent and application problems covering the 160 objectives in sections 1.1–1.8, 2.1–2.6, 3.1–3.9, 4.1–4.5, 5.1–5.5 and 6.1–6.5. The map lists all 38 sections across six alternative components; all eight Pure Mathematics 1 sections and all six Pure Mathematics 2 sections, from Paper 2 Algebra through Numerical solution of equations, currently have full teaching sequences. Paper 3 Algebra, section 3.1, links the six shared foundation lessons plus six lessons on partial fractions and rational binomial expansions; Paper 3 logarithms, trigonometry and numerical equations use the shared lessons, with the Paper 1 graph foundation linked into trigonometry. Paper 3 differentiation combines the shared methods with its inverse-tangent derivative lesson. Paper 3 Integration links four shared standard and identity lessons plus seven advanced lessons, covering all six section 3.5 requirements. Eight Vectors lessons cover all six section 3.7 requirements, with 2D and 3D line and solid applications. Nine Differential equations lessons cover all four section 3.8 requirements, including Paper 3 integration techniques, initial conditions, equilibrium solutions and conditional model interpretations. Eleven Complex numbers lessons cover all eight section 3.9 requirements, with seven Argand diagrams and explicit argument and boundary checks. All nine Pure Mathematics 3 sections now have teaching linked to every mapped requirement. Nine Forces and equilibrium lessons cover all seven section 4.1 requirements, with six force diagrams, signed incline-friction intervals, reaction-dependent pull and push limits and separate-body third-law checks. Nine Kinematics lessons cover the four official section 4.2 requirements, with the graph requirement mapped to three relationships. Six motion diagrams support directed quantities, graph slopes and signed areas, Paper 1 time-calculus methods, reversal distances, constant-acceleration stages and shared-clock particle meetings. Six Momentum lessons cover both section 4.3 requirements, including signed system totals, conservation assumptions, outgoing velocities, coalescence, unknown masses and successive impacts. Four diagrams show system momentum through the stipulated events, and supplied kinetic-energy checks distinguish conservation of momentum from changes in kinetic energy. Twelve Newton’s laws lessons cover all four section 4.4 requirements, with vertical support and release stages, rough horizontal and incline motion, inferred parameters, hanging and surface pulley systems, rigid tow-bar thrust and slack-string transitions. Seven force diagrams support the separate-particle balances. Nine Energy, work and power lessons cover all five section 4.5 requirements, with angle-dependent work, kinetic and reference potential energy, signed net-work balances, smooth curved slides, rough and driven slopes, ideal connected-system energy and instantaneous power on hills. Five diagrams distinguish the energy inputs, losses and valid positive-speed force model. All five Mechanics sections now have teaching and staged practice linked to their mapped requirements. Thirteen Representation of data lessons cover all five section 5.1 requirements, including choice and limitations of displays, ordered and back-to-back stems, raw summaries and box plots, unequal-width density histograms, cumulative-frequency interpolation, descriptive moment calculations, grouped estimates, coding, combining two datasets and correcting totals. Six accessible diagrams retain repeated observations and distinguish numerical spread, frequency area and estimated readings. Ten Permutations and combinations lessons cover both official section 5.2 requirements, with selection and ordered roles, repeated objects, adjacency blocks, non-adjacency gaps, leading-zero cases, labelled multi-row seating and mixed restrictions. Six accessible diagrams show outcome trees, ordered versus unordered choices, repeated copies, blocks, gaps and distinct row seats. Eleven Probability lessons cover all four section 5.3 requirements, with equiprobable elementary outcomes, selection-based probabilities, complements and disjoint regions, independence tests, restricted sample spaces, two-way tables, replacement and removal trees, reversed conditions and unknown parameters. Seven accessible diagrams show dice outcomes, random subsets, event regions, a conditioned grid, a count table and two probability trees. Twelve Discrete random variables lessons cover all three official section 5.4 requirements, with complete probability tables, moment and parameter checks, binomial assumptions, exact probabilities and moments, geometric first-success probabilities, tails, expectation and integer reliability thresholds. Six accessible diagrams distinguish outcome grouping, weighted moments, fixed trials, discrete probability bars, waiting conventions and integer tail deadlines. Twelve Normal distribution lessons cover all three official section 5.5 requirements, with continuous-area interpretation, MF19 cumulative tables, full standardisation, tail and interval regions, conditional probabilities, percentiles, one- and two-parameter inference, binomial suitability, continuity corrections and approximate integer thresholds. Seven accessible diagrams show model areas, table reading, aligned scales, intervals, quantiles, matched binomial and normal shapes and whole integer cells. All five Statistics 1 sections now have teaching and staged practice linked to their mapped requirements. Fourteen Poisson distribution lessons cover all five official section 6.1 requirements, with homogeneous event-count assumptions, exposure scaling, exact masses and tails, conditional count events, moments, parameter inference, probability duration targets, disjoint and overlapping windows, rare-count binomial approximations, matched normal approximations and continuity corrections. Nine accessible diagrams show event exposures, unit conversions, discrete bars, complete tail complements, population moments, shared windows, approximate count models and integer cells. Exact checks distinguish approximate and exact close-target decisions. Fourteen Linear combinations lessons cover the six listed results in official section 6.2, with affine moments, signed scales and constant cases, expectation without independence, independent squared variance contributions, fixed and random fees, independent copies versus reused values, affine normal events, independent normal sums and differences, weighted and conditional areas, coefficient inference, exact independent Poisson sums and overlapping exposure limits. Nine accessible diagrams distinguish scale changes, variance contributions, component costs, copy mechanisms, aligned normal scales, summed areas, Poisson count pairs and unsupported distribution claims. Fourteen Continuous random variables lessons cover both official section 6.3 requirements, with density validity and units, support-aware intervals, complements and conditional regions, first and second moments, percentiles by direct density areas, symmetry checks, parameter inference, infinite-support power and exponential integrals, divergent-moment limits and combined applications. Nine accessible diagrams distinguish density height from probability, normalisation, negative-density failures, central and conditional areas, mean versus median, percentile locations and partial infinite tails. Explicit cumulative distribution function knowledge is not required. Eighteen Sampling and estimation lessons cover all eight official section 6.4 requirements, with population and frame definitions, random-number selection, sampling-quality limits, independent sample-mean moments, normal means, CLT conditions, SE scaling, raw, summarised and coded unbiased estimates, known-variance and large-sample mean intervals, repeated-sampling interpretation, precision planning, proportion intervals, sparse-outcome limits and contextual applications. Ten accessible diagrams distinguish selection frames, number rejection, repeated sample means, population versus mean density, CLT models, variance divisors, interval targets, illustrative coverage, integer size checks and proportion margins. Eighteen Hypothesis tests lessons cover all five official section 6.5 requirements, with pre-specified alternatives, inclusive count tails, explicit equal-tail conventions, integer critical regions and actual significance, exposure-aware Poisson tests, continuity-corrected count approximations, normal and large-sample mean tests, confidence-interval connections, error classifications, fixed-region binomial, Poisson and normal error probabilities and sample-size power comparisons. Nine accessible diagrams show the testing procedure, exact count regions, unequal Poisson tails, discrete significance jumps, corrected count boundaries, matching mean intervals and alternative error distributions. All shared lessons are counted once in the course total. All 38 sections now link teaching and practice. Eight earlier overview lessons remain available, but do not count as full section coverage. This is a content-availability measure, not full programme completion or exam readiness. Written practice is self-checked, not officially marked. Check the 2028–2030 syllabus separately if sitting an examination from 2028; this teaching map uses 2026–2027 requirements.

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