Power rule

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In calculus, the power rule is used to differentiate functions of the form <math>f(x) = x^r</math>, whenever <math>r</math> is a real number. Since differentiation is a linear operation on the space of differentiable functions, polynomials can also be differentiated using this rule. The power rule underlies the Taylor series as it relates a power series with a function's derivatives.

Statement of the power ruleEdit

Let <math>f</math> be a function satisfying <math>f(x)=x^r</math> for all <math>x</math>, where <math>r \in \mathbb{R}</math>.Template:Efn Then,

The power rule for integration states that

for any real number <math>r \neq -1</math>. It can be derived by inverting the power rule for differentiation. In this equation C is any constant.

ProofsEdit

Proof for real exponentsEdit

Let Template:Nowrap where <math>r</math> is any real number.

If Template:Nowrap then Template:Nowrap where <math>\ln</math> is the natural logarithm function, or Template:Nowrap as was required. Therefore, applying the chain rule to Template:Nowrap we see that <math display="block">f'(x)=\frac{r}{x} e^{r\ln x}= \frac{r}{x}x^r</math> which simplifies to Template:Nowrap

When Template:Nowrap we may use the same definition with Template:Nowrap where we now have Template:Nowrap This necessarily leads to the same result. Note that because <math>(-1)^r</math> does not have a conventional definition when <math>r</math> is not a rational number, irrational power functions are not well defined for negative bases. In addition, as rational powers of −1 with even denominators (in lowest terms) are not real numbers, these expressions are only real valued for rational powers with odd denominators (in lowest terms).

Finally, whenever the function is differentiable at Template:Nowrap the defining limit for the derivative is: <math display="block">\lim_{h\to 0} \frac{h^r - 0^r}{h}</math> which yields 0 only when <math>r</math> is a rational number with odd denominator (in lowest terms) and Template:Nowrap and 1 when Template:Nowrap For all other values of Template:Nowrap the expression <math>h^r</math> is not well-defined for Template:Nowrap as was covered above, or is not a real number, so the limit does not exist as a real-valued derivative. For the two cases that do exist, the values agree with the value of the existing power rule at 0, so no exception need be made.

The exclusion of the expression <math>0^0</math> (the case Template:Nowrap from our scheme of exponentiation is due to the fact that the function <math>f(x, y) = x^y</math> has no limit at (0,0), since <math>x^0</math> approaches 1 as x approaches 0, while <math>0^y</math> approaches 0 as y approaches 0. Thus, it would be problematic to ascribe any particular value to it, as the value would contradict one of the two cases, dependent on the application. It is traditionally left undefined.

Proofs for integer exponentsEdit

Proof by induction (natural numbers)Edit

Let <math>n\in\N</math>. It is required to prove that <math>\frac{d}{dx} x^n = nx^{n-1}.</math> The base case may be when <math>n=0</math> or <math>1</math>, depending on how the set of natural numbers is defined.

When <math>n=0</math>, <math>\frac{d}{dx} x^0 = \frac{d}{dx} (1) = \lim_{h \to 0}\frac{1-1}{h} = \lim_{h \to 0}\frac{0}{h} = 0 = 0x^{0-1}.</math>

When <math>n=1</math>, <math>\frac{d}{dx} x^1 = \lim_{h \to 0}\frac{(x+h)-x}{h} = \lim_{h \to 0}\frac{h}{h} = 1 = 1x^{1-1}.</math>

Therefore, the base case holds either way.

Suppose the statement holds for some natural number k, i.e. <math>\frac{d}{dx}x^k = kx^{k-1}.</math>

When <math>n=k+1</math>,<math display="block">\frac{d}{dx}x^{k+1} = \frac{d}{dx}(x^k \cdot x) = x^k \cdot \frac{d}{dx}x + x \cdot \frac{d}{dx}x^k = x^k + x \cdot kx^{k-1} = x^k + kx^k = (k+1)x^k = (k+1)x^{(k+1)-1}</math>By the principle of mathematical induction, the statement is true for all natural numbers n.

Proof by binomial theorem (natural number)Edit

Let <math>y=x^n</math>, where <math>n\in \mathbb{N} </math>.

Then,<math display="block">\begin{align} \frac{dy}{dx} &=\lim_{h\to 0}\frac{(x+h)^n-x^n}h\\[4pt] &=\lim_{h\to 0}\frac{1}{h} \left[x^n+\binom n1 x^{n-1}h+\binom n2 x^{n-2}h^2+\dots+\binom nn h^n-x^n \right]\\[4pt] &=\lim_{h\to 0}\left[\binom n 1 x^{n-1} + \binom n2 x^{n-2}h+ \dots+\binom nn h^{n-1}\right]\\[4pt] &=nx^{n-1} \end{align}</math>

Since n choose 1 is equal to n, and the rest of the terms all contain h, which is 0, the rest of the terms cancel. This proof only works for natural numbers as the binomial theorem only works for natural numbers.

Generalization to negative integer exponentsEdit

For a negative integer n, let <math>n=-m</math> so that m is a positive integer. Using the reciprocal rule,<math display="block">\frac{d}{dx}x^n = \frac{d}{dx} \left(\frac{1}{x^m}\right) = \frac{-\frac{d}{dx}x^m}{(x^m)^2} = -\frac{mx^{m-1}}{x^{2m}} = -mx^{-m-1} = nx^{n-1}.</math>In conclusion, for any integer <math>n</math>, <math>\frac{d}{dx}x^n = nx^{n-1}.</math>

Generalization to rational exponentsEdit

Upon proving that the power rule holds for integer exponents, the rule can be extended to rational exponents.

Proof by chain ruleEdit

This proof is composed of two steps that involve the use of the chain rule for differentiation.

Let <math>y=x^r=x^\frac1n</math>, where <math>n\in\N^+</math>. Then <math>y^n=x</math>. By the chain rule, <math>ny^{n-1}\cdot\frac{dy}{dx}=1</math>. Solving for <math>\frac{dy}{dx}</math>, <math display="block">\frac{dy}{dx}

=\frac{1}{ny^{n-1}} =\frac{1}{n\left(x^\frac1n\right)^{n-1}} =\frac{1}{nx^{1-\frac1n}} =\frac{1}{n}x^{\frac1n-1} =rx^{r-1}</math>Thus, the power rule applies for rational exponents of the form <math>1/n</math>, where <math>n</math> is a nonzero natural number. This can be generalized to rational exponents of the form <math>p/q</math> by applying the power rule for integer exponents using the chain rule, as shown in the next step.

Let <math>y=x^r=x^{p/q}</math>, where <math>p\in\Z, q\in\N^+,</math> so that <math>r\in\Q</math>. By the chain rule, <math display="block">\frac{dy}{dx}

=\frac{d}{dx}\left(x^\frac1q\right)^p =p\left(x^\frac1q\right)^{p-1}\cdot\frac{1}{q}x^{\frac1q-1} =\frac{p}{q}x^{p/q-1}=rx^{r-1}</math> From the above results, we can conclude that when <math>r</math> is a rational number, <math>\frac{d}{dx} x^r=rx^{r-1}.</math>

Proof by implicit differentiationEdit

A more straightforward generalization of the power rule to rational exponents makes use of implicit differentiation.

Let <math> y=x^r=x^{p/q}</math>, where <math>p, q \in \mathbb{Z}</math> so that <math>r \in \mathbb{Q}</math>.

Then,<math display="block">y^q=x^p</math>Differentiating both sides of the equation with respect to <math>x</math>,<math display="block">qy^{q-1}\cdot\frac{dy}{dx} = px^{p-1}</math>Solving for <math>\frac{dy}{dx}</math>,<math display="block">\frac{dy}{dx} = \frac{px^{p-1}}{qy^{q-1}}.</math>Since <math>y=x^{p/q}</math>,<math display="block">\frac d{dx}x^{p/q} = \frac{px^{p-1}}{qx^{p-p/q}}.</math>Applying laws of exponents,<math display="block">\frac d{dx}x^{p/q} = \frac{p}{q}x^{p-1}x^{-p+p/q} = \frac{p}{q}x^{p/q-1}.</math>Thus, letting <math>r=\frac{p}{q}</math>, we can conclude that <math>\frac d{dx}x^r = rx^{r-1}</math> when <math>r</math> is a rational number.

HistoryEdit

The power rule for integrals was first demonstrated in a geometric form by Italian mathematician Bonaventura Cavalieri in the early 17th century for all positive integer values of <math>{\displaystyle n}</math>, and during the mid 17th century for all rational powers by the mathematicians Pierre de Fermat, Evangelista Torricelli, Gilles de Roberval, John Wallis, and Blaise Pascal, each working independently. At the time, they were treatises on determining the area between the graph of a rational power function and the horizontal axis. With hindsight, however, it is considered the first general theorem of calculus to be discovered.<ref name="Boyer">Template:Cite book</ref> The power rule for differentiation was derived by Isaac Newton and Gottfried Wilhelm Leibniz, each independently, for rational power functions in the mid 17th century, who both then used it to derive the power rule for integrals as the inverse operation. This mirrors the conventional way the related theorems are presented in modern basic calculus textbooks, where differentiation rules usually precede integration rules.<ref>Template:Cite book</ref>

Although both men stated that their rules, demonstrated only for rational quantities, worked for all real powers, neither sought a proof of such, as at the time the applications of the theory were not concerned with such exotic power functions, and questions of convergence of infinite series were still ambiguous.

The unique case of <math>r = -1</math> was resolved by Flemish Jesuit and mathematician Grégoire de Saint-Vincent and his student Alphonse Antonio de Sarasa in the mid 17th century, who demonstrated that the associated definite integral,

representing the area between the rectangular hyperbola <math>xy = 1</math> and the x-axis, was a logarithmic function, whose base was eventually discovered to be the transcendental number e. The modern notation for the value of this definite integral is <math>\ln(x)</math>, the natural logarithm.

GeneralizationsEdit

Complex power functionsEdit

If we consider functions of the form <math>f(z) = z^c</math> where <math>c</math> is any complex number and <math>z</math> is a complex number in a slit complex plane that excludes the branch point of 0 and any branch cut connected to it, and we use the conventional multivalued definition <math>z^c := \exp(c\ln z)</math>, then it is straightforward to show that, on each branch of the complex logarithm, the same argument used above yields a similar result: <math>f'(z) = \frac{c}{z}\exp(c\ln z)</math>.<ref>Template:Cite book</ref>

In addition, if <math>c</math> is a positive integer, then there is no need for a branch cut: one may define <math>f(0) = 0</math>, or define positive integral complex powers through complex multiplication, and show that <math>f'(z) = cz^{c-1}</math> for all complex <math>z</math>, from the definition of the derivative and the binomial theorem.

However, due to the multivalued nature of complex power functions for non-integer exponents, one must be careful to specify the branch of the complex logarithm being used. In addition, no matter which branch is used, if <math>c</math> is not a positive integer, then the function is not differentiable at 0.