Expand a positive-integer binomial with correct coefficients and signs
Use factorial and combination notation to build every term of a finite expansion, keeping constants and negative signs inside powers.
Section 1.6 · Series
Pure Mathematics 1 (Paper 1) · AS & A Level Mathematics 9709, 2026–2027. Use positive-integer binomial powers and positive-integer term indices. Written practice is self-checked.
Before you begin
- Multiply powers and simplify integer fractions.
- Expand brackets and distinguish odd and even powers of a negative number.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is (6 choose 2)?
What you will learn
- Interpret n! and the combination coefficient (n choose k).
- Expand (a + b)^n for a positive integer n, including scaled and negative terms.
Count the choices that produce each term
In a product of n identical brackets (a + b), a term with k factors of b and n − k factors of a can arise in (n choose k) ways. The coefficient is n!/[k!(n − k)!] for integers 0 ≤ k ≤ n. Factorial n! means n(n − 1)…2 × 1, and 0! = 1. Thus (5 choose 2) = 5!/(2!3!) = 10.
The term is (n choose k)a^(n − k)b^k. Start with k = 0 and finish with k = n, giving n + 1 terms before collecting any like powers. For n = 4 the coefficients are 1, 4, 6, 4, 1. If b contains a negative sign or a numerical factor, raise its whole bracket to k; the binomial coefficient does not replace that factor.
Track both powers and check the expansion
For (2 − x)^4, take a = 2 and b = −x. The terms are 2⁴, 4(2³)(−x), 6(2²)(−x)², 4(2)(−x)³ and (−x)⁴. They simplify to 16 − 32x + 24x² − 8x³ + x⁴. Odd powers of −x are negative and even powers positive. The exponent of 2 decreases while the exponent of x increases.
A finite positive-integer expansion is exact when every term is retained. Substituting x = 0 checks the constant, and a second input can expose a sign error. If you deliberately keep only the constant and linear terms for a small-change estimate, label that as an approximation and identify the omitted terms. Greatest-term methods and non-integer infinite binomial expansions are outside this section's stated requirements.
Pause and explain
What is the coefficient of x² in (1 − 2x)^4?
Worked example
Expand (2 − x)^4 in ascending powers of x.
Show the worked solution
- Use coefficients 1, 4, 6, 4, 1 and terms 2⁴, 4(2³)(−x), 6(2²)(−x)², 4(2)(−x)³, (−x)⁴.
- Simplify constants and signs to obtain 16 − 32x + 24x² − 8x³ + x⁴.
- At x = 1, the original is 1 and the expansion gives 16 − 32 + 24 − 8 + 1 = 1.
Answer 16 − 32x + 24x² − 8x³ + x⁴.
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.
Expand (1 + 3x)^3 exactly.
Give me a hint
Use the four coefficients 1, 3, 3, 1 and retain the factor 3 inside each power.
Compare my reasoning
- Write 1³ + 3(1²)(3x) + 3(1)(3x)² + (3x)³.
- This becomes 1 + 9x + 27x² + 27x³.
- Check x = 1: the expansion totals 64, matching 4³.
1 + 9x + 27x² + 27x³.
Look for these in your work
- Raise the numerical factor with x in every term.
- Check the exact expansion by substitution.
Expand (3 − 2x)^3 in ascending powers of x.
Give me a hint
Choose a = 3 and b = −2x; do not separate the minus sign from its power.
Compare my reasoning
- Use 3³ + 3(3²)(−2x) + 3(3)(−2x)² + (−2x)³.
- Simplify to 27 − 54x + 36x² − 8x³.
- At x = 1 the expansion gives 1, matching (3 − 2)³; both odd-power terms are negative.
27 − 54x + 36x² − 8x³.
Look for these in your work
- Track powers of both the constant and the variable term.
- Apply signs consistently to odd and even powers.
A supplied volume model is V(h) = (2 + h)^3 for −0.2 ≤ h ≤ 0.2. Expand it, find V(0.1), and explain the difference from the linear estimate 8 + 12h.
Give me a hint
Keep every term for the exact polynomial before comparing with the truncated model.
Compare my reasoning
- The exact expansion is 8 + 12h + 6h² + h³.
- At h = 0.1, V = 8 + 1.2 + 0.06 + 0.001 = 9.261.
- The linear estimate is 9.2, so it is smaller by 0.061, exactly the omitted 6h² + h³ at this input.
V(h) = 8 + 12h + 6h² + h³; V(0.1) = 9.261; the linear estimate is lower by 0.061.
Look for these in your work
- Separate exact expansion from a labelled approximation.
- Identify and calculate the omitted contribution at an allowed input.
Common mistakes
- The numerical factor inside b must also be raised to the term's power.
- The first term uses k = 0; the expansion has n + 1 terms.
- A truncated estimate is different from the exact finite expansion.
What can you explain now?
Find the coefficient of x³ in (2 − x)^4.
Compare with the explanation
−8.
The k = 3 term is 4 × 2 × (−x)³, so its coefficient is −8.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain why a binomial coefficient counts choices of brackets, then reconstruct an expansion with a negative scaled variable term and check it at two inputs.
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.