Differentiate rational powers, sums and differences
Rewrite roots and reciprocals as powers, apply the power rule term by term and check where the real function has a finite derivative.
Pure Mathematics 1 (Paper 1) · AS & A Level Mathematics 9709, 2026–2027. Use an informal understanding of limits; formal general differentiation from first principles and points of inflexion are not required in section 1.7. Written practice is self-checked.
Before you begin
- Use exponent laws for roots and reciprocals.
- Understand first- and second-derivative notation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which power equals 1/x² for x ≠ 0?
What you will learn
- Differentiate constant multiples, sums and differences of xⁿ for rational n.
- Find first and second derivatives while respecting roots, reciprocals and exceptional inputs.
Multiply by the exponent, then subtract one
For a rational exponent n at inputs where the real function is differentiable, d(xⁿ)/dx = nx^(n − 1). Multiply a constant coefficient by n and decrease the power by 1. A constant term has derivative zero. Differentiate sums and differences term by term: the sign belongs to its coefficient and must remain in the calculation.
For x > 0, y = 3x³ − 4√x + 5/x + 7 becomes 3x³ − 4x^(1/2) + 5x^(−1) + 7. Its first derivative is 9x² − 2x^(−1/2) − 5x^(−2), or 9x² − 2/√x − 5/x². Differentiating again gives 18x + x^(−3/2) + 10x^(−3). Keep negative exponents separate from negative coefficients.
Check the original domain and any derivative exclusions
A reciprocal requires a non-zero denominator, and an even root needs a non-negative radicand for a real value. The derivative of √x is 1/(2√x) for x > 0; its endpoint x = 0 does not have a finite derivative. Do not extend a formula to an input where its original function is undefined or where its derivative expression has a zero denominator.
Odd roots can be real at negative inputs. For y = x^(1/3), interpret x^(1/3) as the real cube root; y′ = 1/[3x^(2/3)] at x ≠ 0. At x = −8, x^(2/3) = (∛(−8))² = 4, so the gradient is 1/12. At zero, nearby chord gradients h^(1/3)/h = 1/|h|^(2/3) grow without bound, so no finite derivative exists there. Check the particular real function rather than excluding every negative input automatically.
Pause and explain
What is the derivative of −4√x for x > 0?
Worked example
For x > 0, find the first and second derivatives of y = 3x³ − 4√x + 5/x + 7.
Show the worked solution
- Rewrite the variable terms as 3x³ − 4x^(1/2) + 5x^(−1).
- Apply the power rule to obtain y′ = 9x² − 2x^(−1/2) − 5x^(−2).
- Apply it again: y″ = 18x + x^(−3/2) + 10x^(−3); the supplied positive domain makes these real and finite.
Answer y′ = 9x² − 2/√x − 5/x²; y″ = 18x + 1/x^(3/2) + 10/x³, for x > 0.
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.
For y = 2x⁴ − 3x² + 6x − 9, find y′, y″ and y′ at x = 2.
Give me a hint
Differentiate each term independently, then evaluate the first-derivative rule.
Compare my reasoning
- The first derivative is y′ = 8x³ − 6x + 6; the constant −9 contributes zero.
- The second derivative is y″ = 24x² − 6.
- At x = 2, y′ = 64 − 12 + 6 = 58.
y′ = 8x³ − 6x + 6; y″ = 24x² − 6; y′(2) = 58.
Look for these in your work
- Apply the coefficient and exponent changes to every term.
- Distinguish differentiating from evaluating a rule.
For x > 0, y = 4√x − 6/x + 3. Find y′, y″ and their values at x = 4 exactly.
Give me a hint
Rewrite the root and reciprocal as rational powers before differentiating twice.
Compare my reasoning
- y = 4x^(1/2) − 6x^(−1) + 3 gives y′ = 2/√x + 6/x².
- Differentiate again to obtain y″ = −1/x^(3/2) − 12/x³.
- At x = 4, y′ = 1 + 3/8 = 11/8 and y″ = −1/8 − 3/16 = −5/16.
y′ = 2/√x + 6/x²; y″ = −1/x^(3/2) − 12/x³; y′(4) = 11/8; y″(4) = −5/16.
Look for these in your work
- Track the reciprocal coefficient's sign through both differentiations.
- Use exact root and power values at the permitted input.
For the real cube-root curve y = x^(1/3), find its gradient at x = −8 and explain what happens to the derivative at x = 0.
Give me a hint
Interpret the odd root as a real cube root, then check the exceptional zero input separately.
Compare my reasoning
- For x ≠ 0, y′ = 1/[3x^(2/3)].
- At x = −8, x^(2/3) = (−2)² = 4, so y′ = 1/12.
- At zero, chord gradients 1/|h|^(2/3) become unbounded as h tends to zero, so there is no finite derivative there.
Gradient 1/12 at x = −8; no finite derivative at x = 0.
Look for these in your work
- Evaluate the rational power using the real odd root.
- Check the exceptional input instead of substituting into an undefined derivative formula.
Common mistakes
- A negative exponent is not a negative coefficient.
- Differentiate a constant to zero rather than keeping it in the gradient.
- Check real domains and finite derivatives at root or reciprocal boundaries.
What can you explain now?
Find the derivative of 6/x² for x ≠ 0.
Compare with the explanation
−12/x³.
Write 6x^(−2), multiply its coefficient by −2 and decrease the exponent to −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
Rewrite a root and a reciprocal as powers, differentiate twice and explain how you decide which real inputs permit each rule.
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.