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# functions that are their own derivative | Spanglers – General Blog News

17 #### The spinoff of the exponential operate with base 2

With a purpose to take the spinoff of the exponential operate, say start{align*} f(x)=2^x finish{align*} we could also be tempted to make use of the ability rule. Nonetheless, the exponential operate \$2^x\$ could be very completely different from the ability \$x^2\$, as a result of in \$2^x\$ the variable \$x\$ itself is within the exponent. We might get into quite a lot of bother with utterly bogus outcomes if we changed the spinoff of \$2^x\$ with \$x 2^{x-1}\$. Do not do that.

To find out the spinoff of the exponential operate, we have to return to the restrict definition of the spinoff. In keeping with the restrict definition, start{align} f'(x) &= lim_{h to 0}frac{f(x+h) – f(x)}{h}notag &= lim_{h to 0} frac{2^{x+h}-2^x}{h}. label{limit_definition_2_to_x} finish{align} Right here we used \$h\$ for the step measurement as an alternative of \$Delta x\$, however it does not matter what we name it.

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The scenario may look hopeless, till we keep in mind the foundations for exponents. We will change \$2^{x+h}\$ with \$2^x 2^h\$ within the numerator. Then, we will issue \$2^{x}\$ out of the numerator since \$2^{x}2^h-2^x = 2^x(2^h-1)\$. In actual fact, we will do even higher than that. The issue \$2^x\$ does not even rely upon \$h\$, so we’re allowed to drag it out of the restrict. Our ensuing simplified kind for the spinoff of \$2^x\$ is start{align} f'(x) &= 2^x lim_{h to 0} frac{2^{h}-1}{h}. label{limit_number} finish{align}

Did we make any progress? We calculated that the spinoff of the operate \$f(x)=2^x\$ is the same as the operate itself (\$2^x\$) occasions a mysterious trying expression involving a restrict. However, the mysterious expression involving a restrict is only a single quantity. Nothing extra, nothing much less. What quantity is it? It is an irrational quantity, however you may estimate it your self by calculating \$(2^h-1)/h\$ for smaller and smaller values of \$h\$.

Use the next applet to estimate \$\$lim_{h to 0} frac{2^{h}-1}{h}.\$\$ Sort in smaller and smaller values of \$h\$ within the first column, and it’ll show \$(2^h-1)/h\$ within the second column, calculated utilizing the values of \$h\$ you entered. Whenever you hold making \$h\$ smaller and smaller, you may discover that finally, the primary 4 digits of \$(2^h-1)/h\$ do not change any extra. These 4 unchanging digits are an estimate of the quantity \$\$lim_{h to 0} frac{2^{h}-1}{h}\$\$ correct to 4 decimal locations. Since \$h\$ might be constructive or detrimental, additionally examine detrimental values of \$h\$ to make it possible for \$(2^h-1)/h\$ approaches the identical quantity as \$h\$ approaches zero from beneath.

Nonetheless, we will do even higher than this. Have a look at the restrict definition of equation eqref{limit_definition_2_to_x} for \$f'(x)\$, and plug in \$x=0\$. What do you get? You get precisely start{align} f'(0) = lim_{h to 0} frac{2^{h}-1}{h}. finish{align} Our mysterious quantity in equation eqref{limit_number} for the spinoff \$f'(x)\$ is simply the spinoff itself evaluated at \$x=0\$. We will rewrite expression eqref{limit_number} for the spinoff of \$f(x)=2^x\$ as start{align} f'(x) &=f(x) f'(0). label{derivative_f_fprime} finish{align}

Equation eqref{derivative_f_fprime} states that the spinoff \$f'(x)\$ is simply equal to the operate \$f(x)\$ itself, multiplied by our mysterious quantity, which can also be the slope \$f'(0)\$ of the tangent line at \$x=0\$. It is a unusual conclusion, so we higher affirm that it’s actually true.

Use the next applet to persuade your self that the result’s legitimate. Test that the spinoff \$f'(x)\$ is known as a a number of of the operate \$f(x)\$ by verifying that the ratio \$f'(x)/f(x)\$ does not change as you alter \$x\$. This ratio needs to be \$f'(0)\$, which needs to be the mysterious quantity you estimated to 4 digits in addition to the slope of the tangent line when \$x=0\$.

Assuming you’ve got verified that equation eqref{derivative_f_fprime} appears to be right, we have achieved all we will for the spinoff of \$f(x)=2^x\$. The result’s simply expressed by way of the mysterious quantity \$\$f'(0) = lim_{h to 0} frac{2^h-1}{h},\$\$ however it’s just a few irrational quantity which you’ve got estimated.

#### Exponential features with different bases

Does the end result that we obtained for \$f(x)=2^x\$ generalize to different bases in addition to base 2? See if you will get related outcomes for the exponential operate \$\$g(x)=b^x\$\$ the place the bottom \$b\$ is a constructive parameter.

First, repeat the identical analytic calculation, beginning with the restrict definition and changing 2 with \$b\$. You must receive that start{align*} g'(x) &=g(x) g'(0) finish{align*} the place start{align*} g'(0) = lim_{h to 0} frac{b^h-1}{h}. finish{align*}

Second, decide two different values of \$b\$ in addition to \$b=2\$. Attempt one worth of \$b\$ lower than one and one worth of \$b\$ better than one, so that you experiment with each exponential decay and exponential progress. For every of your selections for \$b\$, use the primary applet to estimate the primary 4 digits of the worth of the mysterious restrict expression defining \$g'(0)\$. Then, use the second applet to confirm that this worth is each the spinoff \$g'(0)\$ at zero and the ratio between the spinoff \$g'(x)\$ and the operate \$g(x)\$.

#### The exponential operate with base \$e\$

As it is best to have skilled, the mysterious restrict expression \$\$lim_{h to 0} frac{b^h-1}{h}\$\$ is fairly annoying. Certain it’s only a quantity, however it’s some loopy irrational quantity that’s completely different for every worth of the bottom \$b\$. Everytime you’d wish to calculate the spinoff of \$b^x\$ for one more base \$b\$, you’d should undergo the identical calculation to find out the mysterious issue. That is inconvenient, and, furthermore, you’d should provide you with some good option to talk your end result to others.

Let’s take a barely completely different strategy. We’ll use the truth that \$\$lim_{h to 0} frac{b^h-1}{h}\$\$ modifications with the bottom \$b\$ to our benefit. Let’s discover a worth of \$b\$ for which the mysterious issue is so simple as attainable: the worth 1. Utilizing both the primary applet with a really small \$h\$ or the second applet, experiment with completely different values of \$b\$ to discover a explicit worth of \$b\$ the place \$\$lim_{h to 0} frac{b^h-1}{h} approx 1.\$\$

Had been you capable of finding such a quantity? It turns on the market is just one worth of \$b\$ for which the mysterious issue is strictly one. We denote this quantity by \$e\$, which is outlined to be the one quantity for which \$\$lim_{h to 0} frac{e^h-1}{h} = 1.\$\$ Since \$e\$ is irrational, we can’t have the ability to get an actual illustration of this quantity utilizing these strategies. However, in the event you had been actually affected person and had a software like these applets that would estimate the restrict with adequate accuracy, you would estimate \$e\$ to 30 digits, and decide that \$\$e approx 2.71828182845904523536028747135.\$\$ The above applets perceive \$e\$, so you may sort in \$e\$ for \$b\$ in both applet and see that the mysterious restrict is certainly one for this case.

One other option to see that is to plot the mysterious restrict as a operate of \$b\$ and to watch that it crosses one when \$b=e\$. We will conclude that for \$b=e\$, the spinoff of the operate \$\$g(x)=e^x,\$\$ evaluated at zero is \$g'(0)=1\$. Because of this the spinoff of \$g\$ is strictly the operate itself: start{align*} g'(x)=g(x). finish{align*} For that reason, solely while you enter \$e\$ for \$b\$ within the second applet do the operate and its spinoff precisely coincide.

This elementary property of \$e^x\$, that it’s its personal spinoff, is without doubt one of the causes that \$e\$ is the most typical base used for the exponential operate all through the sciences. The operate \$e^x\$ is sometimes called merely the exponential operate.

#### Including parameters to the exponential operate

When discussing the exponential operate, we launched just a few parameters, writing the overall exponential operate as \$\$f(x)=cb^{kx}\$\$ with parameters \$b\$, \$c\$, and \$okay\$. We then confirmed how the parameters \$b\$ and \$okay\$ had been redundant, so we solely wanted one in every of them. Since we discovered \$b=e\$ makes the derivatives good, let’s simply use the bottom \$e\$ and write our exponential features as \$\$f(x)=ce^{kx},\$\$ which has simply two parameters \$c\$ and \$okay\$.

Do the parameters \$c\$ and \$okay\$ change the properties of the exponential operate? Up to now, we have simply appeared on the case the place \$c=1\$ and \$okay=1\$ and located that \$f'(x)=f(x)\$. Is \$f'(x)=f(x)\$ for different values of \$c\$ and \$okay\$? You should use the second applet, above, to discover this query. Test the “extra choices” examine field, which is able to reveal packing containers during which to vary \$c\$ and \$okay\$. You’ll be able to observe if the graph of \$f\$ and \$f’\$ are nonetheless an identical and if the ratio \$f’/f\$ continues to be one when \$b=e\$ however \$c\$ and \$okay\$ are completely different values. If the ratio \$f'(x_0)/f(x_0)\$ modifications while you change \$c\$ or \$okay\$, is the worth nonetheless impartial of \$x_0\$? (In different phrases, is \$f’\$ nonetheless a a number of of \$f\$?) How does the ratio rely upon \$c\$ and \$okay\$?

To acquire a definitive reply on the spinoff of \$f(x)=ce^{kx}\$, we will repeat the above calculation utilizing the restrict definition. start{align*} f'(x_0) &= lim_{h to 0} frac{f(x_0+h) – f(x_0)}{h} &= lim_{h to 0} frac{ce^{okay(x_0+h)}-ce^{kx_0}}{h} finish{align*} Once more, the foundations for exponents assist us simplify this restrict. Since \$e^{okay(x_0+h)} = e^{kx_0+kh} = e^{kx}e^{kh}\$, we will issue out a \$ce^{kx_0}\$ from the numerator: \$ce^{kx_0}e^{kh}-ce^{kx_0} = ce^{kx_0}(e^{kh} -1)\$. The quantity \$ce^{kx_0}\$ is similar factor as \$f(x_0)\$. It does not rely upon \$h\$, so we will pull it out of the restrict and conclude that start{align*} f'(x_0) &= f(x_0) lim_{h to 0} frac{e^{kh}-1}{h}. finish{align*}

Apart from the issue of \$okay\$ within the exponent, we have now precisely the identical expression as earlier than. The parameter \$c\$ does not change the end result (as presumably you decided from exploring the applet). The parameter \$okay\$ does change issues, although, because it alters the mysterious restrict expression. Can we cut back this new mysterious restrict again into the previous one?

We will get our previous mysterious restrict again with only one manipulation. Let’s outline a brand new amount to be equal to the exponent in our new expression: \$w = kh\$. Since \$h=w/okay\$, we will rewrite the fraction contained in the restrict as \$\$frac{e^{w}-1}{w/okay} = kfrac{e^{w}-1}{w}.\$\$ For the reason that parameter \$okay\$ is just a few quantity, the brand new amount \$w=kh\$ goes to zero simply as \$h\$ goes to zero. So, we will change the restrict \$h to 0\$ with the equal restrict \$w to 0\$. We will rewrite the brand new mysterious restrict as start{align*} lim_{h to 0} frac{e^{kh}-1}{h} &= okay lim_{w to 0} frac{e^{w}-1}{w}. finish{align*}

However wait, the \$w\$ restrict is strictly the identical as our unique myterious restrict. OK, it’s written by way of \$w\$ moderately than the unique \$h\$. However that’s simply notation; it does not matter what image we use for the amount that goes to zero within the restrict. All the next expressions are precisely the identical quantity. start{align*} lim_{h to 0} frac{e^h-1}{h} &= lim_{w to 0} frac{e^{w}-1}{w} &=lim_{bigstar to 0} frac{e^bigstar-1}{bigstar} finish{align*} And what quantity are these expressions equal to once more? Bear in mind the definition of the quantity \$e\$. It’s precisely the quantity that makes these limits equal to 1.

Subsequently start{align*} lim_{h to 0} frac{e^{kh}-1}{h} &= okay lim_{w to 0} frac{e^{w}-1}{w} = okay cdot 1 = okay, finish{align*} and the spinoff of \$f(x)=ce^{kx}\$ is start{align*} f'(x_0) &= f(x_0) lim_{h to 0} frac{e^{kh}-1}{h} = kf(x_0). finish{align*} It is that easy, and also you most likely figured this out already from exploring the applet. Whenever you differentiate \$e^{kx}\$, you simply get the operate again once more, multiplied by the parameter \$okay\$.

Let’s write this end result utilizing some fancy notation. Recall that we will write the spinoff \$f'(x)\$ additionally as \$diff{f}{x}(x)\$. We will separate this into \$diff{}{x} f(x)\$. The \$diff{}{x}\$ by itself means to take the spinoff of no matter comes after it, the place \$x\$ is the variable. (We regularly say “taking the spinoff with respect to \$x\$.”) Utilizing this \$diff{}{x}\$ notation, we will write our end result for the spinoff of \$e^{kx}\$ as start{align} diff{}{x} e^{kx} = ke^{kx}. label{derivative_ekx}tag{5} finish{align}

#### What is that this mysterious restrict?

We have now only one extra matter of enterprise earlier than we have wrapped up our exploration of the spinoff of the exponential operate. What concerning the spinoff of exponential features with base \$b\$ equal to a quantity in addition to \$e\$? In keeping with our calculations, the spinoff of \$g(x)=b^x\$ is the same as \$g'(x) =g(x) g'(0)\$, the place the issue \$g'(0)\$ was the mysterious restrict start{align*} g'(0) = lim_{h to 0} frac{b^h-1}{h}. finish{align*} Let’s have a look at if we will discover a nicer trying expression for that restrict. It would nonetheless most likely be some irrational quantity, however we’ll search for a greater option to calculate this expression than making an attempt to judge the restrict instantly.

The reply is one easy trick. We will flip any exponential base \$b\$ into the nicer exponential base \$e\$. We simply should keep in mind that the inverse of the exponential operate \$e^x\$ is the logarithm base \$e\$, or the pure logarithm. We regularly write the pure logarithm of \$x\$ merely as \$log x\$, as a result of if you’re going to take a logarithm and nobody tells you in any other case, you may as effectively do the pure factor and use base \$e\$. To be completely specific, we will additionally use the notation \$ln x\$ for the pure logarithm, which we’ll use for now.

Since \$ln x\$ and \$e^x\$ are inverses, it follows that for any quantity \$x>0\$, we will additionally write \$x\$ as \$\$x = e^{ln x}.\$\$ If as an alternative of \$x\$, we insert the expression \$b^x\$, we will rewrite \$b^x\$ as \$\$b^x = e^{ln b^x} = e^{x ln b},\$\$ the place for the final step, we used one of many fundamental guidelines for logarithms: \$log b^x = x log b\$. We have decided that \$b^x\$ is similar factor as \$e^{kx}\$ the place \$okay=ln b\$. This explains why \$okay\$ and \$b\$ are redundant parameters of the exponential operate. We need not use \$b^x\$ with completely different bases \$b\$, we may get the identical operate by utilizing \$e\$ for the bottom and writing the operate as \$e^{kx}\$ the place \$okay=ln b\$.

Provided that \$okay=ln b\$, we will use method eqref{derivative_ekx} for the spinoff of \$e^{kx}\$ to compute the spinoff of \$b^x\$. We simply must multiply the operate by \$okay\$, which is similar factor as multiply by \$ln b\$. Our revised method for the spinoff of \$b^x\$ is start{align*} diff{}{x} b^x = b^x ln b. finish{align*}

Lastly we have found out what the mysterious restrict was. The mysterious restrict was merely \$ln b\$ in disguise: start{align*} lim_{h to 0} frac{b^h -1}{h} = ln b. finish{align*}

In case you are not comfy with these manipulations, it is best to do the most effective to persuade your self that this restrict is the pure logarithm. Try it out on a calculator. Whenever you enter \$ln 2\$, do you get a end result much like the estimate you made above for the restrict \$lim_{h to 0} (2^h-1)/h\$? Have a look at how the graph of \$(b^h-1)/h\$ for various values of \$h\$ converges \$ln b\$ as \$h\$ will get smaller and smaller.

It is lots nicer to have a method for the spinoff of \$b^x\$ with out the mysterious restrict, utilizing \$ln b\$ as an alternative. Often, in calculus, although, we’ll avoid bases one other than the pure one \$e\$. Crucial of the formulation that we have derived are \$\$diff{}{x}e^x = e^x\$\$ and its generalization \$\$diff{}{x}e^{kx} = ke^{kx}.\$\$ (You’ll be able to all the time multiply both method by a relentless quantity \$c\$.)

In actual fact, \$e^{x}\$ (together with its multiples \$ce^{x}\$) is the one operate that’s its personal spinoff. So, in the event you had been informed to discover a operate \$f(x)\$ the place \$f'(x)=f(x)\$, you’d know that the operate have to be \$f(x)=ce^x\$ for some (unknown) quantity \$c\$.

The worksheet on exploring the spinoff of the exponential operate accommodates inquiries to information you thru discovering the properties of the exponential operate spinoff.