What Is The Constant Multiple Rule?
The Constant Multiple Rule is the rule that lets a constant factor pass through differentiation or integration unchanged. If $c$ is a constant and $f$ is a differentiable (or integrable) function, then the constant comes out to the front and the operation acts only on $f$.
For differentiation:
$$\frac{d}{dx}\big[c,f(x)\big] = c,f'(x)$$
For integration:
$$\int c,f(x),dx = c\int f(x),dx$$
In words: to differentiate or integrate a constant times a function, deal with the function and keep the constant riding along in front. The rule is one half of what makes differentiation and integration linear operations; the other half is the sum and difference rule, which splits a sum into separate pieces. Together they let you break almost any polynomial or combination into terms you can handle one at a time.
One warning up front, because it drives the rest of this article: the factor that slides out has to be a genuine constant. A variable such as $x$ cannot be pulled out the same way, and treating one as if it could is the most common error on this topic.
Why Does The Constant Factor Out Of A Derivative?
The cleanest way to see the rule is to prove it from the definition of the derivative. Let $g(x) = c,f(x)$. The limit definition gives:
$$g'(x) = \lim_{h \to 0} \frac{g(x+h) - g(x)}{h} = \lim_{h \to 0} \frac{c,f(x+h) - c,f(x)}{h}$$
Factor the constant $c$ out of the numerator:
$$g'(x) = \lim_{h \to 0} \frac{c\big[f(x+h) - f(x)\big]}{h} = \lim_{h \to 0} c\cdot\frac{f(x+h) - f(x)}{h}$$
Now the single step that carries the whole rule: because $c$ does not change as $h \to 0$, the constant-multiple law of limits lets it come out in front of the limit.
$$g'(x) = c\cdot\lim_{h \to 0} \frac{f(x+h) - f(x)}{h} = c,f'(x)$$
That limit is exactly $f'(x)$, so $g'(x) = c,f'(x)$. The proof leans on nothing more than the fact that a constant is unaffected by the limit process, which is why the rule feels almost too simple to need a proof.
Geometrically, multiplying $f$ by $c$ stretches its graph vertically by a factor of $c$. Stretching the height by $c$ multiplies the steepness of every tangent line by $c$ as well, so the slope function scales by the same $c$. The algebra and the picture say the same thing.
Why Does The Constant Factor Out Of An Integral?
Integration inherits the rule for the same reason, and there are two honest ways to see it.
From the antiderivative. Suppose $F$ is an antiderivative of $f$, meaning $F'(x) = f(x)$. Differentiate $c,F(x)$ using the differentiation rule just proved:
$$\frac{d}{dx}\big[c,F(x)\big] = c,F'(x) = c,f(x)$$
So $c,F(x)$ is an antiderivative of $c,f(x)$. Writing that as an indefinite integral gives the rule, with the constant of integration added because antiderivatives are only fixed up to a constant:
$$\int c,f(x),dx = c,F(x) + C = c\int f(x),dx$$
From the Riemann sum. For a definite integral, the same factoring happens inside the sum that defines area. Every sampled height $f(x_i)$ is scaled by $c$, and a constant pulls straight out of a finite sum and then out of its limit:
$$\int_a^b c,f(x),dx = \lim_{n \to \infty} \sum_{i=1}^{n} c,f(x_i),\Delta x = c\lim_{n \to \infty} \sum_{i=1}^{n} f(x_i),\Delta x = c\int_a^b f(x),dx$$
Geometrically, scaling the curve's height by $c$ scales the area beneath it by $c$: a region three times as tall holds three times the area over the same base. This is one of the standard properties of definite integrals, and it is the reason you can lift a coefficient out before hunting for an antiderivative.
How Do You Use The Constant Multiple Rule? Worked Examples
Each example is fully stepped. Every integral is checked by differentiating the antiderivative back to the original integrand.
Example 1: Differentiate a constant times a power.
Find $\dfrac{d}{dx}\big[5x^3\big]$.
The constant $5$ rides out front; differentiate $x^3$ with the power rule:
$$\frac{d}{dx}\big[5x^3\big] = 5\cdot\frac{d}{dx}\big[x^3\big] = 5\cdot 3x^2 = 15x^2$$
Final answer: $15x^2$.
Example 2: Differentiate a constant times a trig function.
Find $\dfrac{d}{dx}\big[7\sin x\big]$.
The constant $7$ stays in front; the derivative of $\sin x$ is $\cos x$:
$$\frac{d}{dx}\big[7\sin x\big] = 7\cos x$$
Final answer: $7\cos x$.
Example 3: Integrate a constant times a power.
Evaluate $\displaystyle\int 6x^2,dx$.
Pull the $6$ out, integrate $x^2$, then keep the constant of integration:
$$\int 6x^2,dx = 6\int x^2,dx = 6\cdot\frac{x^3}{3} + C = 2x^3 + C$$
Check: $\dfrac{d}{dx}\big[2x^3\big] = 6x^2$, the original integrand, so the antiderivative is correct.
Final answer: $2x^3 + C$.
Example 4: Integrate a constant times a trig function.
Evaluate $\displaystyle\int -4\cos x,dx$.
Pull the $-4$ out, integrate $\cos x$ to $\sin x$:
$$\int -4\cos x,dx = -4\int \cos x,dx = -4\sin x + C$$
Check: $\dfrac{d}{dx}\big[-4\sin x\big] = -4\cos x$, the integrand, so the result is valid.
Final answer: $-4\sin x + C$.
Example 5: A definite integral.
Evaluate $\displaystyle\int_0^{2} 10x,dx$.
Pull the $10$ out, integrate, then apply the limits:
$$\int_0^{2} 10x,dx = 10\int_0^{2} x,dx = 10\left[\frac{x^2}{2}\right]_0^{2} = 10\big(2 - 0\big) = 20$$
Check: $\displaystyle\int_0^{2} 10x,dx = \big[5x^2\big]_0^{2} = 20 - 0 = 20$, which matches.
Final answer: $20$.
What Does The Constant Multiple Rule Look Like For Common Functions?
Because the constant simply rides along, the rule pairs neatly with the standard derivatives and integrals. This one table settles the routine cases (here $c$ is any constant).
Table: The Constant Multiple Rule applied to the functions you meet most often.
Function $f(x)$ | $c,f(x)$ | Derivative $\dfrac{d}{dx}\big[c,f(x)\big]$ | Integral $\displaystyle\int c,f(x),dx$ |
|---|---|---|---|
$x^n$ (with $n \neq -1$) | $c,x^n$ | $c,n,x^{n-1}$ | $c\cdot\dfrac{x^{n+1}}{n+1} + C$ |
$\sin x$ | $c\sin x$ | $c\cos x$ | $-c\cos x + C$ |
$\cos x$ | $c\cos x$ | $-c\sin x$ | $c\sin x + C$ |
$e^x$ | $c,e^x$ | $c,e^x$ | $c,e^x + C$ |
$\dfrac{1}{x}$ | $\dfrac{c}{x}$ | $-\dfrac{c}{x^2}$ | $c\ln\lvert x\rvert + C$ |
Read across any row and the pattern is identical: whatever the derivative or integral of $f$ is, the answer for $c,f$ is that same result with $c$ in front. A fuller catalogue lives in the table of derivatives.
Why Does The Constant Multiple Rule Work?
The rule is a direct consequence of linearity, and it helps to separate the intuition from the algebra.
A constant is deaf to the variable. Differentiation and integration are both built on limits, and a limit only reacts to the part of an expression that moves with the variable. A constant sits still, so it slips outside the limit rather than getting caught in it.
Scaling a graph scales its slope. Multiplying every output of $f$ by $c$ stretches the curve vertically by $c$. Two points that were a certain height apart are now $c$ times as far apart over the same horizontal gap, so every slope is multiplied by $c$. That is $c,f'(x)$ seen as a picture.
Scaling a graph scales its area. The area under $c,f$ over an interval is $c$ times the area under $f$, because every thin strip is $c$ times as tall over the same width. That is $c\int f$ seen as a picture.
Seen this way, the differentiation face and the integration face are not two rules to memorise. They are one fact, that a constant factors out of a limit, applied to the two operations calculus is built from.
Who Discovered And Proved The Constant Multiple Rule?
The rule was used long before anyone worried about proving it. The founders of calculus treated "pull the constant out" as obvious; rigour came a century and a half later.
Two names anchor the timeline:
Gottfried Wilhelm Leibniz (1646–1716, Germany) gave calculus the $\int$ sign and the $dx$ notation, the very symbols that make "factor the constant out front" something you can see on the page.
Augustin-Louis Cauchy (1789–1857, France) turned the derivative and integral into limit-based definitions, which is what let the Constant Multiple Rule be proved rather than merely assumed.
Where Is The Constant Multiple Rule Used In The Real World?
The rule is quiet but everywhere, because unit conversions and scaling factors are constants riding in front of a changing quantity.
Physics: doubling a mass doubles the force needed for the same acceleration, since $F = ma$ scales linearly; rates of change scale with the constant right alongside the quantity.
Engineering and signals: an amplifier with gain $c$ multiplies a signal by a constant, so the rate at which the output changes is $c$ times the rate of the input, a direct reading of $\frac{d}{dx}[c,f]=c,f'$.
Economics: if every unit sells at a fixed price $p$, total revenue is $p$ times quantity, and the marginal (rate) and total (area) both carry that constant factor out front.
Chemistry: a reaction rate written as $k$ times a concentration term keeps the rate constant $k$ outside the calculus, so scaling the concentration scales the modelled rate by the same factor.
Everyday scaling: doubling a recipe, a blueprint, or a budget multiplies every rate and every running total by the same number, which is the Constant Multiple Rule applied without the notation.
One small rule lets every field pull its unit factors and scaling constants out of the way and focus the calculus on the part that actually changes.
What Are The Most Common Mistakes With The Constant Multiple Rule?
These four errors account for most lost marks, and each matches a question real students ask on r/calculus, r/learnmath, and AP Calculus 2.6 study guides.
Dropping the constant coefficient.
Where it slips in:
A student differentiates $5x^3$, focuses on the $x^3$, writes $3x^2$, and forgets the $5$ was ever there.
Don't do this:
Do not differentiate the function and leave the coefficient behind. The constant stays as a multiplier on the answer.
The correct way:
Keep the constant in front the whole time: $\dfrac{d}{dx}\big[5x^3\big] = 5\cdot 3x^2 = 15x^2$.
Zeroing the coefficient as if it were a standalone constant.
Where it slips in:
A student remembers that the derivative of a constant is $0$, then wrongly kills the coefficient too, writing $\dfrac{d}{dx}\big[7x\big] = 0$.
Don't do this:
Do not confuse a constant term with a constant coefficient. A lone $7$ has derivative $0$; a $7$ multiplying $x$ does not vanish.
The correct way:
Apply the rule: $\dfrac{d}{dx}\big[7x\big] = 7\cdot\dfrac{d}{dx}\big[x\big] = 7\cdot 1 = 7$.
Pulling a variable out as if it were a constant.
Where it slips in:
Facing $\dfrac{d}{dx}\big[x\sin x\big]$, a student treats the leading $x$ like the constant in the rule and writes $x\cos x$.
Don't do this:
Do not factor a variable out of a derivative or integral. There is no "constant multiple rule" for a variable factor, because $x$ changes with the limit and will not come out.
The correct way:
Use the product rule when both factors depend on $x$: $\dfrac{d}{dx}\big[x\sin x\big] = \sin x + x\cos x$.
Forgetting the constant of integration.
Where it slips in:
A student pulls a constant out of an indefinite integral, integrates, and stops at $2x^3$ without the $+C$.
Don't do this:
Do not drop the constant of integration on an indefinite integral. Factoring $c$ out does not remove the family of antiderivatives.
The correct way:
Write $\displaystyle\int 6x^2,dx = 2x^3 + C$. The $+C$ stays until definite limits are applied.
Practice Problems On The Constant Multiple Rule
Work each one, then check against the answer. Answers are verified.
Find $\dfrac{d}{dx}\big[8x^5\big]$.
(Answer: $8\cdot 5x^4 = 40x^4$.)Find $\dfrac{d}{dx}\big[-3\cos x\big]$.
(Answer: $-3\cdot(-\sin x) = 3\sin x$.)Evaluate $\displaystyle\int 12x^3,dx$.
(Answer: $12\cdot\dfrac{x^4}{4} + C = 3x^4 + C$.)Evaluate $\displaystyle\int -5e^x,dx$.
(Answer: $-5e^x + C$.)Evaluate $\displaystyle\int_1^{3} 4x,dx$.
(Answer: $4\left[\dfrac{x^2}{2}\right]_1^{3} = 4(4) = 16$.)Find $\dfrac{d}{dx}\left[\dfrac{2}{3}\sqrt{x}\right]$.
(Answer: $\dfrac{2}{3}\cdot\dfrac{1}{2}x^{-1/2} = \dfrac{1}{3\sqrt{x}}$.)
Where Should You Go Next After The Constant Multiple Rule?
The Constant Multiple Rule is a first building block, and several natural doors open from here.
Rules of differentiation. See how the constant multiple and sum rules combine into the full linearity toolkit for polynomials.
The power rule. The partner rule you used in almost every example above, for differentiating $x^n$.
Methods of integration. Once the constant is out front, these techniques handle the function that remains.
If your child is meeting the Constant Multiple Rule for the first time, a live Bhanzu trainer teaches it from the limit definition up, so the rule feels earned rather than memorised, in the Bhanzu math program.
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