Unit 2Derivative of xnx^n
Concept 4.224Built for May 2027 AP Exam

Derivative of xnx^n

Reviewed by ACEpath AP Math Editorial Team

Last Updated: September 2026

30-second summary

The power rule is stated for a constant exponent, and "constant" is broader than "whole number". ddxxn=nxn−1\frac{d}{dx}x^n = nx^{n-1} holds for every real nn, which means roots and reciprocals are already covered once they are rewritten as powers: x=x1/2\sqrt{x} = x^{1/2} and 1x3=x−3\frac{1}{x^3} = x^{-3}. The derivation does not assume nn is an integer either. Expand (x+h)n(x+h)^n, and every term after the first carries a factor of hh, so the limit leaves exactly nxn−1nx^{n-1}. Two consequences matter on the exam. The rewrite is the work, not the differentiation. And a fractional power can fail at the endpoint of its domain, where the slope becomes infinite rather than finite.

On the AP Exam

What you’ll see
A term with a radical, a reciprocal, or a power of a power, inside a rate, a tangent slope, or a second derivative.
What you’ll do
Convert to xnx^n, differentiate with one step, then simplify back into the form the answer choices or the rubric expect.
Memory hook
A root is a fraction, a reciprocal is a negative. Costume first, rule second.
Use it on the exam
For MCQs: Options are frequently the same expression written two ways, one with a radical and one with a fractional exponent. Simplify back to the original costume before you compare. For FRQs: The rubric accepts a derivative in either form, but a later part usually substitutes a number. A negative exponent is easier to evaluate than a compound fraction, so convert toward whichever is easier to plug into.

Concept Review

Same Rule, Different Costumes

Figure 1 — Each row is one term in three states. The middle column is the only real work: once the term is written as a power, the derivative follows in a single step and the rule itself never changes.

Three rows mapping a radical or reciprocal in its original form to a fractional or negative power and then to its derivative, showing that the power rule is unchanged once the rewrite is done

The Rewrites Worth Memorising

Written asAs a powerDerivativeDerivative, original form
x\sqrt{x}x1/2x^{1/2}12x−1/2\frac{1}{2}x^{-1/2}12x\frac{1}{2\sqrt{x}}
x3\sqrt[3]{x}x1/3x^{1/3}13x−2/3\frac{1}{3}x^{-2/3}13x23\frac{1}{3\sqrt[3]{x^2}}
1x\frac{1}{x}x−1x^{-1}−x−2-x^{-2}−1x2-\frac{1}{x^2}
1x4\frac{1}{x^4}x−4x^{-4}−4x−5-4x^{-5}−4x5-\frac{4}{x^5}
xxx\sqrt{x}x3/2x^{3/2}32x1/2\frac{3}{2}x^{1/2}3x2\frac{3\sqrt{x}}{2}

The right-hand column is what the answer key usually prints, and the two middle columns are what you actually compute. Translating between them is a skill in itself, and it is the one that decides whether an otherwise correct derivative matches an option.

Where the Rule Comes From

Expand (x+h)n(x+h)^n with the binomial theorem and subtract xnx^n:

(x+h)n−xn=nxn−1h+n(n−1)2xn−2h2+⋯(x+h)^n - x^n = nx^{n-1}h + \frac{n(n-1)}{2}x^{n-2}h^2 + \cdots

Divide by hh and every term after the first still carries a factor of hh:

lim⁡h→0(x+h)n−xnh=nxn−1+0+⋯=nxn−1\lim_{h \to 0}\frac{(x+h)^n - x^n}{h} = nx^{n-1} + 0 + \cdots = nx^{n-1}

The exponent drops because exactly one of the nn factors is replaced by the increment, and there are nn ways to choose it. Nothing in that counting argument requires nn to be a whole number, which is why the fractional case inherits the rule rather than needing a separate proof.

Where a Fractional Power Breaks

For f(x)=xf(x) = \sqrt{x} the derivative 12x\frac{1}{2\sqrt{x}} is undefined at x=0x = 0, and the graph agrees: the tangent there is vertical, so no finite slope exists. This is the vertical-tangent failure from the previous topic, appearing inside a formula instead of a picture. The lesson is that the rule reports a slope wherever the rewritten power is differentiable, and the endpoint of a root's domain has to be checked separately.

Common Misconception Check

  • Myth 1: "The power rule only applies to positive integer exponents." Truth: It applies to every constant exponent. The integer case is the easiest, not the only one.
  • Myth 2: "The derivative of x\sqrt{x} is 12x\frac{1}{2\sqrt{x}} by a separate rule." Truth: It is the same rule after one rewrite. Memorising it separately doubles the material for no gain.
  • Myth 3: "A negative exponent can be left in the denominator." Truth: It can, but the minus sign must be tracked. Most lost points in this topic come from dropping it during the translation back.

Topic-Specific Study Method

Rewrite, Then Drop

Borrowed idea: Rewrite before you differentiate. Every root and every reciprocal is a power wearing a costume. Turn the root into a fractional exponent, turn the reciprocal into a negative one, and the rule runs unchanged. Does the exponent have to be a whole number? No, the same algebra works for any constant, which is why the rewrite is worth the ten seconds. What about the endpoint where a root starts? There the slope may be infinite, and the derivative simply does not exist. So check where the expression lives before you report a number.

You seeRewriteDerivativeWatch for
x\sqrt{x}x1/2x^{1/2}12x−1/2\frac{1}{2}x^{-1/2}slope infinite at 00
1x\frac{1}{\sqrt{x}}x−1/2x^{-1/2}−12x−3/2-\frac{1}{2}x^{-3/2}sign and domain
5x2\frac{5}{x^2}5x−25x^{-2}−10x−3-10x^{-3}coefficient survives
x2xx^2\sqrt{x}x5/2x^{5/2}52x3/2\frac{5}{2}x^{3/2}add the exponents first
x3x\frac{x^3}{x}x2x^22x2xsimplify before differentiating

① Write ten expressions in mixed costumes, convert each to a single power, and differentiate. Then convert every answer back to the original costume and check that the two forms agree at a test value.

② Differentiate x\sqrt{x} three ways: by the rule, by the definition with a conjugate, and by reading the graph at x=4x = 4. All three should give 14\frac{1}{4}.

③ Find where f(x)=x2/3f(x) = x^{2/3} has a finite derivative and where it does not. The formula gives 23x−1/3\frac{2}{3}x^{-1/3}, which is undefined at 00, and that single point is the cusp from the previous topic.

Training & Error Diagnosis

Core Archetype (Simplest Form): Differentiate f(x)=3x−2xf(x) = 3\sqrt{x} - \frac{2}{x}.

Standard Execution:

  1. Rewrite. f(x)=3x1/2−2x−1f(x) = 3x^{1/2} - 2x^{-1}.
  2. Differentiate. f′(x)=32x−1/2+2x−2f'(x) = \frac{3}{2}x^{-1/2} + 2x^{-2}.
  3. Translate back. f′(x)=32x+2x2f'(x) = \frac{3}{2\sqrt{x}} + \frac{2}{x^2}.
  4. Check. At x=1x = 1 both forms give 3.53.5, and the original function is decreasing in its reciprocal term, which matches the positive second term.

Exam Variation (Context / Parameter Shift): Water drains from a tank so that the depth at time tt minutes is D(t)=129−tD(t) = 12\sqrt{9 - t} centimeters for 0≤t<90 \le t < 9. Find the rate at which the depth is changing at t=5t = 5, with units, and explain what happens as t→9−t \to 9^-.

Solution: Rewrite as D(t)=12(9−t)1/2D(t) = 12(9-t)^{1/2}, so D′(t)=6(9−t)−1/2⋅(−1)=−69−tD'(t) = 6(9-t)^{-1/2} \cdot (-1) = -\frac{6}{\sqrt{9-t}}. At t=5t = 5 this gives −3-3 centimeters per minute. As t→9−t \to 9^- the denominator tends to zero, so the rate becomes unbounded: the tank empties with a vertical tangent, which is the endpoint failure again.

Common Traps & Failure Signals:

  1. The index as exponent (Signal: x\sqrt{x} differentiated to 12x1\frac{1}{2}x^{1} or similar): the rewrite used the wrong power.
  2. The lost sign (Signal: a positive answer where the function is decreasing): the negative exponent was translated back without its minus.
  3. The unexamined endpoint (Signal: a finite slope reported at the start of a root): the domain was never checked.

6D Cognitive Error Diagnosis (Where Your Thinking Broke):

  • Root Unrewritten: You never turned the root into a power.
  • Sign Lost →\rightarrow Carry the minus sign through the translation back.
  • Endpoint Ignored: You reported a finite slope at an input where the rewritten power has a negative exponent over a vanishing quantity, which is the formula's way of saying the tangent is vertical and no derivative exists there.

Past 10-Year AP Exam Appearance

Testing Frequency
Rare in FRQs
Fractional and negative exponents usually arrive already rewritten, and no released part from 2017 to 2026 scores the rewrite as an object of its own
10-Year Total Count
1 of 9 Exams · 2 FRQ Parts
Both parts are 2019, and in both the rewritten power is the only route to the slope
FRQ Years Appeared
2019 (Q4b, Q5a)

Practice Examples

MCQ 1

A Fractional Power

If f(x)=x3/2f(x) = x^{3/2}, what is f′(x)f'(x)?

MCQ 2

A Reciprocal Root

If g(x)=1x3g(x) = \frac{1}{\sqrt[3]{x}}, what is g′(x)g'(x)?

MCQ 3

Why It Still Works

Why does the power rule apply to x1/2x^{1/2} even though the exponent is not a whole number?

FRQ 1 • A Tank That Empties

Water drains from a tank so that the depth of water at time tt minutes is modeled by D(t)=129−tD(t) = 12\sqrt{9 - t} centimeters for 0≤t<90 \le t < 9.

(a) Write D(t)D(t) as a power of (9−t)(9 - t) and find D′(t)D'(t).

(b) Find the rate at which the depth is changing at t=5t = 5 minutes. Include units.

(c) Explain why D′(9)D'(9) does not exist, using the form of D′(t)D'(t) rather than the graph.

(d) A second tank drains so that its depth is E(t)=12(9−t)1/3E(t) = 12(9-t)^{1/3}. Compare E′(5)E'(5) with D′(5)D'(5) and explain which tank is draining faster at that instant.

Cheatsheet

Derivative of xnx^n

RewriteDerivative
xnx^nnonenxn−1nx^{n-1}
x\sqrt{x}x1/2x^{1/2}12x−1/2\frac{1}{2}x^{-1/2}
xn\sqrt[n]{x}x1/nx^{1/n}1nx1/n−1\frac{1}{n}x^{1/n - 1}
1xn\frac{1}{x^n}x−nx^{-n}−nx−n−1-nx^{-n-1}
xaxbx^a x^bxa+bx^{a+b}(a+b)xa+b−1(a+b)x^{a+b-1}
xaxb\frac{x^a}{x^b}xa−bx^{a-b}(a−b)xa−b−1(a-b)x^{a-b-1}
TOPIC REVIEW • NEXT STEP IN YOUR LEARNING LOOP

Ready to check your understanding?

Strengthen this topic with AP-style practice, or continue to next topic.