Connect graph intersections to simultaneous equations
Find complete intersection coordinates and use the resulting equation to decide whether graphs meet or touch.
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
- Solve simultaneous linear equations and quadratics.
- Use a discriminant to classify genuine quadratic equations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What must an intersection point satisfy?
What you will learn
- Find intersections of two lines or of a line and a quadratic curve.
- Interpret repeated or absent real solutions as geometric intersection conditions.
A common point satisfies both equations
At an intersection, the same pair (x, y) belongs to both graphs. Solve their equations simultaneously, then give both coordinates. For 3x + 2y = 14 and x − y = 1, write y = x − 1 and substitute to obtain 5x − 2 = 14. This gives x = 16/5 and y = 11/5; substitute into both lines to verify.
Two distinct parallel lines have no intersection; coincident lines have infinitely many. For example y = 2x + 1 and y = 2x − 3 would require 1 = −3, so they do not meet. Do not apply a quadratic discriminant to this inconsistent linear equation; classify the actual equation you obtain.
Substitution explains what a sketch suggests
For y = x² − 4x + 1 and y = 2x − 4, equating y-values gives x² − 6x + 5 = 0, with roots 1 and 5. Recover the corresponding line outputs −2 and 6 to get (1, −2) and (5, 6). Roots of the curve's original x-intercept equation would answer a different question.
A line and a genuine quadratic curve meet twice when their intersection equation has positive discriminant, touch when it has a repeated root and do not meet when there are no real roots. If a parameter changes the equation, apply the condition before solving for coordinates. A sketch guides interpretation, while substitution provides exact values and checks completeness.
Pause and explain
An intersection quadratic has discriminant zero. What does this mean for a line and parabola?
Worked example
Find the intersection of 3x + 2y = 14 and x − y = 1.
Show the worked solution
- Rearrange the second equation to y = x − 1.
- Substitute: 3x + 2(x − 1) = 14, giving x = 16/5.
- Then y = 11/5; check 48/5 + 22/5 = 14 and 16/5 − 11/5 = 1.
Answer Intersection (16/5, 11/5).
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
Find the intersection of y = 2x − 1 and y = −x + 8.
Give me a hint
Equate the two y-expressions, then recover y.
Compare my reasoning
- Write 2x − 1 = −x + 8.
- Solve 3x = 9 to get x = 3.
- Both lines give y = 5 at x = 3, so the pair is verified.
(3, 5).
Look for these in your work
- Solve a single equation for the common input.
- Check the recovered output in both lines.
Find all intersections of y = x² − 4x + 1 and y = 2x − 4.
Give me a hint
Rearrange the equality of y-values into a quadratic equal to zero.
Compare my reasoning
- Equate to obtain x² − 6x + 5 = 0.
- Factor (x − 1)(x − 5) = 0, giving x = 1 or 5.
- Use the line to obtain y = −2 or 6 respectively; check both in the curve.
(1, −2) and (5, 6).
Look for these in your work
- Retain both real roots.
- Pair each input with its own common output.
For which real k does the horizontal line y = k intersect, touch or miss y = x² + 2x + 3?
Give me a hint
Use either the vertex form or the intersection discriminant.
Compare my reasoning
- The curve is y = (x + 1)² + 2, with minimum 2.
- The intersection equation is (x + 1)² = k − 2.
- For k > 2 there are two real inputs; k = 2 gives the tangent point (−1, 2); k < 2 gives none.
Two intersections for k > 2; tangent at (−1, 2) for k = 2; none for k < 2.
Look for these in your work
- Use the intersection equation to classify all cases.
- Locate the tangent point when equality holds.
Common mistakes
- An intersection needs a complete coordinate pair.
- Use the equation obtained by equating the graphs, not either graph's x-intercept equation.
- Distinguish parallel, coincident and quadratic intersection cases.
What can you explain now?
Do the lines y = 2x + 1 and y = 2x − 3 intersect?
Compare with the explanation
No; they are distinct parallel lines.
Their gradients agree, but equating their outputs gives the impossible statement 1 = −3.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Explain how the equation of intersection connects an algebraic root to a point on two graphs. Give examples of no intersection for both parallel lines and a line with a parabola.
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.