3.1 Product Ruleap Calculus

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Consider the product of two simple functions, say$ds f(x)=(x^2+1)(x^3-3x)$. An obvious guess for the derivative of $f$ isthe product of the derivatives of the constituent functions:$ds (2x)(3x^2-3)=6x^3-6x$. Is this correct? We can easily check, byrewriting $f$ and doing the calculation in a way that is known towork. First, $ds f(x)=x^5-3x^3+x^3-3x=x^5-2x^3-3x$, and then$ds f'(x)=5x^4-6x^2-3$. Not even close! What went 'wrong'? Well,nothing really, except the guess was wrong.

So the derivative of $f(x)g(x)$ is NOT as simple as$f'(x)g'(x)$. Surely there is some rule for such a situation? Thereis, and it is instructive to 'discover' it by trying to do thegeneral calculation even without knowing the answer in advance.$$eqalign{{dover dx}(&f(x)g(x)) = lim_{Delta x to0} {f(x+Delta x)g(x+Delta x) - f(x)g(x)over Delta x}cr&=lim_{Delta x to0} {f(x+Delta x)g(x+Delta x)-f(x+Delta x)g(x) + f(x+Delta x)g(x)- f(x)g(x)over Delta x}cr &=lim_{Delta x to0} {f(x+Delta x)g(x+Delta x)-f(x+Delta x)g(x)over Delta x} + lim_{Delta x to0} {f(x+Delta x)g(x)- f(x)g(x)over Delta x}cr &=lim_{Delta x to0} f(x+Delta x){ g(x+Delta x)-g(x)over Delta x} + lim_{Delta x to0} {f(x+Delta x)- f(x)over Delta x}g(x)cr &=f(x)g'(x) + f'(x)g(x)cr}$$A couple of items here need discussion. First, we used a standardtrick, 'add and subtract the same thing', to transform what we hadinto a more useful form. After some rewriting, we realize that we havetwo limits that produce $f'(x)$ and $g'(x)$. Of course, $f'(x)$ and$g'(x)$ must actually exist for this to make sense.We also replaced$ds lim_{Delta xto0}f(x+Delta x)$ with $f(x)$—why is this justified?

What we really need to know here is that $ds lim_{Delta xto 0}f(x+Delta x)=f(x)$, or in the language ofsection 2.5, that $f$ is continuousat $x$. We already know that $f'(x)$ exists (or the whole approach,writing the derivative of $fg$ in terms of $f'$ and $g'$, doesn't makesense). This turns out to imply that $f$ is continuous as well. Here'swhy:$$eqalign{lim_{Delta xto 0} f(x+Delta x) &= lim_{Delta xto 0} (f(x+Deltax) -f(x) + f(x))cr&= lim_{Delta xto 0} {f(x+Delta x) -f(x)over Delta x}Delta x +lim_{Delta xto 0} f(x)cr&=f'(x)cdot 0 + f(x) = f(x)cr}$$

To summarize: the product rule says that$${dover dx}(f(x)g(x)) = f(x)g'(x) + f'(x)g(x).$$

Returning to the example we started with, let $ds f(x)=(x^2+1)(x^3-3x)$.Then $ds f'(x)=(x^2+1)(3x^2-3)+(2x)(x^3-3x)=3x^4-3x^2+3x^2-3+2x^4-6x^2=5x^4-6x^2-3$, as before. In this case it is probably simpler tomultiply $f(x)$ out first, then compute the derivative; here's anexample for which we really need the product rule.

3.1

Example 3.3.1 Compute the derivative of $ds f(x)=x^2sqrt{625-x^2}$. We havealready computed $ds {dover dx}sqrt{625-x^2}={-xoversqrt{625-x^2}}$. Now$$f'(x)=x^2{-xoversqrt{625-x^2}}+2xsqrt{625-x^2}={-x^3+2x(625-x^2)over sqrt{625-x^2}}={-3x^3+1250xover sqrt{625-x^2}}.$$

Exercises 3.3

In 1–4, find the derivatives of the functions using the product rule.

Game 26: november 14 2014 the initials game show. Ex 3.3.1$ds x^3(x^3-5x+10)$(answer)

Ex 3.3.2$ds (x^2+5x-3)(x^5-6x^3+3x^2-7x+1)$(answer)

Ex 3.3.3$ds sqrt{x}sqrt{625-x^2}$(answer)

Ex 3.3.4$displaystyle {sqrt{625-x^2}over x^{20}}$(answer)

3.1 Product Ruleap Calculus

Ex 3.3.5Use the product rule to compute the derivative of $ds f(x)=(2x-3)^2$. Sketch the function. Find an equation of the tangent line to the curve at $x=2$. Sketch the tangent line at $x=2$.(answer)

3.13.1 product ruleap calculus formulas

Ex 3.3.6Suppose that $f$, $g$, and $h$ are differentiable functions.Show that $(fgh)'(x) = f'(x) g(x)h(x) + f(x)g'(x) h(x) + f(x) g(x)h'(x)$.

Ruleap

Ex 3.3.7State and prove a rule to compute $(fghi)'(x)$, similar to the rule in the previous problem.

3.1 Product Ruleap Calculus 14th Edition

Remark 3.3.2 {Product notation}Suppose $ds f_1 , f_2 , ldots f_n$ are functions.The product of all these functions can be written$$ prod _{k=1 } ^n f_k.$$This is similar to the use of $ds sum$ to denote a sum.For example,$$prod _{k=1 } ^5 f_k =f_1 f_2 f_3 f_4 f_5$$and$$prod _ {k=1 } ^n k = 1cdot 2 cdot ldots cdot n = n!.$$We sometimes use somewhat more complicated conditions; for example$$prod _{k=1 , kneq j } ^n f_k$$denotes the product of $ds f_1$ through $ds f_n$ except for $ds f_j$.For example, $$prod _{k=1 , kneq 4} ^5 x^k = xcdot x^2 cdot x^3 cdot x^5 =x^{11}.$$

3.1 Product Ruleap Calculus Calculator

Ex 3.3.8The generalized product rule says that if $ds f_1 , f_2 ,ldots ,f_n$ are differentiable functions at $x$ then$${dover dx}prod _{k=1 } ^n f_k(x) = sum _{j=1 } ^n left(f'_j (x) prod _{k=1 , kneq j} ^n f_k (x)right).$$Verify that this is the same as your answer to the previous problemwhen $n=4$,and write out what this says when $n=5$.

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