Binomial coefficients
Select the required term without expanding every part of a power.
Pure Mathematics 1 (Paper 1) · Mathematics 9709, 2026–2027. This short revision lesson covers coefficients in positive-integer binomial expansions; it does not replace the full section 1.6 pathway. Written solutions use self-check criteria, not an official mark scheme. Specialist accuracy review has not been completed.
Before you begin
- Apply powers to a whole product, including a negative coefficient.
- Calculate combinations and distinguish a term from its numerical coefficient.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is (−3x)², and why?
What you will learn
- Use combinations to identify a term.
- Distinguish a coefficient from the full term.
Why combinations appear
In (a+b)ⁿ, each factor contributes either a or b. A term with r copies of b arises by choosing those r factors from n, giving the multiplier C(n,r). The remaining n−r factors contribute a.
For a positive integer n, the expansion has n+1 terms. To find the coefficient of a particular power of x, identify r and include every numerical factor attached to x.
Keep the whole term together
In (a+bx)ⁿ the term containing xʳ is C(n,r)aⁿ⁻ʳbʳxʳ. The coefficient omits xʳ but includes bʳ. A negative b changes the sign of odd-powered terms.
Sources for this lesson
Pause and explain
Which expression gives the coefficient of x³ in (2 − 3x)^5?
Worked example
Find the coefficient of x² in (3+2x)⁴.
Show the worked solution
- Choose r=2, so C(4,2)=6.
- The remaining constant factor is 3²=9.
- Include 2² from (2x)²: 6×9×4=216.
Answer 216
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 coefficient of x² in (1 − 4x)^5, then write the complete x² term.
Give me a hint
Select two variable factors. Keep the numerical coefficient separate from the power of x.
Compare my reasoning
- The required term uses r = 2: C(5,2) × 1³ × (−4x)².
- C(5,2) = 10 and (−4)² = 16, so the numerical product is 160.
- The coefficient is 160. Restoring the variable factor gives the complete term 160x².
Coefficient: 160; complete term: 160x².
Look for these in your work
- Choose r = 2 from the requested exponent.
- Include the squared numerical factor and its positive sign.
- Distinguish the number 160 from the term 160x².
Find the coefficient of x³ in (3 − 2x)^4. Explain its sign and check it by multiplying (3 − 2x)² by itself.
Give me a hint
For the direct calculation, choose three variable factors and one constant factor. In the check, look for powers adding to 3.
Compare my reasoning
- The direct coefficient is C(4,3) × 3 × (−2)³ = 4 × 3 × (−8) = −96.
- Squaring once gives (3 − 2x)² = 9 − 12x + 4x².
- In the product of these two quadratics, the x³ contributions are (−12x)(4x²) and (4x²)(−12x). Their coefficients total −48 − 48 = −96.
−96; three negative variable factors give a negative coefficient.
Look for these in your work
- Include the remaining constant factor 3.
- Cube the entire coefficient −2.
- Check both contributions to x³ in the product.
In (2 + 3x²)^4, find the coefficients of x⁴ and x³. Explain why the fourth term of the expansion is not the x⁴ term.
Give me a hint
The variable power in the r term is 2r. Count term positions from r = 0.
Compare my reasoning
- For x⁴, solve 2r = 4 to get r = 2. The coefficient is C(4,2) × 2² × 3² = 216.
- All variable powers are even: 0, 2, 4, 6 and 8. There is no x³ term, so its coefficient is 0.
- The x⁴ term has r = 2 and is third when ordered by increasing powers. The fourth term has r = 3 and contains x⁶.
x⁴ coefficient: 216; x³ coefficient: 0. The fourth term contains x⁶.
Look for these in your work
- Match 2r to the requested exponent before selecting a combination.
- Explain an absent power rather than treating it as a missing calculation.
- Distinguish exponent, selection index r and term position r + 1.
Common mistakes
- Do not forget to raise the coefficient of x to the selected power.
- The coefficient is a number, not a number followed by x².
What can you explain now?
Find the coefficient of x in (2−x)³.
Compare with the explanation
−12
The linear term is C(3,1)×2²×(−x)=−12x.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without notes, explain the roles of C(n,r), a^(n−r) and b^r in the coefficient of x^r in (a + bx)^n. Then explain what changes when the variable factor is bx², and when a negative b gives a negative term.
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.