Gradient graphs, concavity and inflection tests
| English | Français |
|---|---|
| point of inflection/pɔɪnt ɒv ɪnˈflekʃn/ | point d'inflexion |
Two curves can both have zero slope, yet one turns and the other keeps rising. Which gradient graph tells the difference?
- Two curves can both have zero slope, yet one turns and the other keeps rising. Which gradient graph tells the difference?
- This lesson studies point of inflection 拐点: A point where a continuous curve changes its direction of concavity.
Choose the mathematical structure
- The sign of f′ identifies increasing or decreasing intervals. Zeros of f′ are stationary candidates. A change + to − gives a local maximum; − to + gives a local minimum. The second derivative f″ measures how the gradient changes: positive means an increasing gradient and a convex (concave-up) curve; negative means a decreasing gradient and a concave (concave-down) curve. An inflection needs a change of concavity, not only f″=0.
- State the allowed inputs and units before calculating. An equation should express the relationship, not just record a calculator entry.
Which description correctly defines point of inflection?
A point where a continuous curve changes its direction of concavity.
Work through a checked case
- Check the result against the starting quantities. Substitute into the original relation, or compare the graph and numerical answer where appropriate.
For f=x³−3x, f′=3x²−3 and f″=6x. The gradient graph is above zero for x<−1 or x>1 and below for −1<x<1. Thus (−1,2) is a local maximum and (1,−2) a local minimum. The second derivative changes from negative to positive at x=0, so (0,0) is an inflection with nonzero gradient −3. For g=x³, g′=3x² stays positive on both sides of zero: (0,0) is a stationary inflection, not a turning point. For q=x⁴, q′=4x³ changes − to + and q″=12x² is nonnegative: (0,0) is a minimum, but not an inflection even though q″(0)=0.
Gradient graphs, concavity and inflection tests
The sign of f′ identifies increasing or decreasing intervals
Connect a derivative calculation to its limit or gradient sign interpretation.
For x³−3x, find the local maximum y-coordinate.
At x=−1 the derivative changes + to − and f(−1)=2.
Test a tempting shortcut
- Do not confuse the height of f with its gradient f′. A decreasing curve can have an increasing gradient while staying below zero in the gradient graph. The test f″(a)=0 is inconclusive; inspect signs on each side. A point of inflection need not be stationary, and a stationary point need not be an extremum. Endpoint extrema on a restricted interval also require checking the endpoints.
- When a shortcut fails, identify the assumption it breaks. Keep an exact value until the requested final rounding.
Every point at which the second derivative is zero is a point of inflection. This claim is false. Explain which definition or assumption it violates.
For x³−3x, find f″(1).
Differentiate twice: f″=6x, so f″(1)=6.
Every point at which the second derivative is zero is a point of inflection.
Do not confuse the height of f with its gradient f′. A decreasing curve can have an increasing gradient while staying below zero in the gradient graph. The test f″(a)=0 is inconclusive; inspect signs on each side. A point of inflection need not be stationary, and a stationary point need not be an extremum. Endpoint extrema on a restricted interval also require checking the endpoints.
Interpret a new situation
- Align the x-axes of the original curve and gradient graph. Mark zeros of f′ and build sign intervals before naming extrema. Read concavity from whether the gradient graph rises or falls. For a twice-differentiable polynomial, use an actual sign change of f″ to establish an inflection and evaluate the original function for its coordinates.
- A complete solution gives the mathematical result and explains what it means. Check that it is possible in the stated context.
For x⁴, find q″(0).
q″=12x² is zero at zero; this alone does not imply an inflection.
Match each part of a complete solution to its purpose.
An assumption justifies the model; a check tests the result; interpretation connects it to the question.
Use this in your course
- 7357 · A-level · G. Match the target tier and specification before assigning extensions.
- Give the method before the final answer, and use the paper's calculator and formula rules. Review a wrong answer by locating the first invalid step.
A point where a continuous curve changes its direction of concavity. Choose the relationship, show the method, check its assumptions and interpret the result.