Solve quadratic equations with a method you can justify
Choose factorisation, a completed square or the quadratic formula, preserving both roots and exact values.
Pure Mathematics 1 (Paper 1) · AS & A Level Mathematics 9709, 2026–2027. Paper 1 is part of every route. Written practice is self-checked; it does not award official examination marks.
Before you begin
- Factorise simple quadratic expressions and use the zero-product rule.
- Complete a square and calculate a discriminant.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
If (x − 2)(x + 5) = 0, which roots follow?
What you will learn
- Solve genuine quadratic equations by the three required methods.
- Retain both algebraic roots before applying a model's domain.
Put every term on one side
The zero-product rule applies to a product equal to zero. Rearrange to ax² + bx + c = 0, keeping a ≠ 0, then look for factors. For 3x² + x − 2, the factorisation (3x − 2)(x + 1) gives roots 2/3 and −1. Cancelling a factor containing x can remove a root, so solve each factor instead.
A completed square is useful when the vertex form is convenient: x² − 4x − 1 = 0 becomes (x − 2)² = 5, then x − 2 = ±√5. The plus-and-minus sign belongs to taking a square root of the equation; the principal square root √5 itself is non-negative.
The formula covers unfactorable real cases
Use x = (−b ± √D)/(2a) after identifying signed coefficients. The whole numerator is divided by 2a. If D > 0 but is not a perfect square, retain a simplified exact surd when exact form is requested. For a numerical answer, keep intermediate precision and round only at the end.
In a model, first find both algebraic roots, then explain which meet the domain. A negative time may solve the quadratic while being outside a model stated for t ≥ 0. Do not silently discard it during the algebra, and do not assume that every physical model requires x to be positive unless its context supplies that restriction.
Pause and explain
What follows from (x − 2)² = 5?
Worked example
Solve 3x² + x − 2 = 0 and check both roots.
Show the worked solution
- Factor as (3x − 2)(x + 1) = 0.
- Solve 3x − 2 = 0 or x + 1 = 0 to get x = 2/3 or −1.
- Substitution gives 4/3 + 2/3 − 2 = 0 and 3 − 1 − 2 = 0.
Answer x = 2/3 or x = −1.
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.
Solve x² − 10x + 16 = 0 by factorisation.
Give me a hint
Find two numbers with product 16 and sum −10.
Compare my reasoning
- The numbers −2 and −8 have the required product and sum.
- Write (x − 2)(x − 8) = 0 and set each bracket to zero.
- Check x = 2 and 8 in the original equation: both give zero.
x = 2 or x = 8.
Look for these in your work
- State a correct factorisation.
- Keep both roots and verify them.
Solve 2x² − 7x + 2 = 0, giving exact roots.
Give me a hint
The discriminant is positive but not a perfect square.
Compare my reasoning
- Identify a = 2, b = −7 and c = 2, giving D = 49 − 16 = 33.
- Use the quadratic formula with numerator 7 ± √33 and denominator 4.
- Retain both exact roots; substituting the formula's values satisfies the quadratic.
x = (7 − √33)/4 or x = (7 + √33)/4.
Look for these in your work
- Apply the denominator to the whole numerator.
- Retain exact surds and both solutions.
A supplied height model is h(t) = −5t² + 20t + 1 metres for t ≥ 0. When does the object reach ground level? Give the time to 3 significant figures.
Give me a hint
Set h(t) = 0 and justify rejecting a time outside the domain.
Compare my reasoning
- Set the height to zero and multiply by −1: 5t² − 20t − 1 = 0.
- The formula gives t = (20 ± √420)/10 = (10 ± √105)/5.
- The minus root is negative; the plus root is about 4.04939 s, so the permitted ground time rounds to 4.05 s.
t = 4.05 s; the negative root lies outside t ≥ 0.
Look for these in your work
- Solve the zero-height equation rather than its derivative.
- Explain the domain rejection and round the final positive time only.
Common mistakes
- Use the zero-product rule only after the product equals zero.
- Include ± when taking square roots of an equation.
- Divide the entire formula numerator by 2a and justify any domain rejection.
What can you explain now?
Solve x² − 4x − 1 = 0 in exact form.
Compare with the explanation
x = 2 − √5 or x = 2 + √5.
Completing the square gives (x − 2)² = 5, so there are two square-root branches.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Name a reason to choose each of factorisation, completing the square and the quadratic formula. Explain when a root can be rejected in a model.
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.