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I'm trying to differentiate the equation below but I fear there must have been an error made. I can't seem to reconcile to the correct answer. The problem comes from James Stewart's Calculus Early Transcendentals, 7th Ed., Page 223, Exercise 25.

Please differentiate $y=\ln(x+\sqrt{1+x^2})$

My Answer:

Differentiate using the natural log rule: $$y'=\left(\frac{1}{x+(1+x^2)^{1/2}}\right)\cdot\left(x+(1+x^2)^{1/2}\right)'$$

Now to differentiate the second term, note the chain rule applied and then simplification:

$$\left(x+(1+x^2)^{1/2}\right)'=1+\frac{1}{2}\cdot(1+x^2)^{-1/2}\cdot(2x)$$

$$1+\frac{1}{2}\cdot(1+x^2)^{-1/2}\cdot(2x)=1+\frac{x}{(1+x^2)^{1/2}}$$

Our expression is now:

$$y'=\left(\frac{1}{x+(1+x^2)^{1/2}}\right)\cdot\left(1+\frac{x}{(1+x^2)^{1/2}}\right)$$

Distribute the left term across the two right terms for my result:

$$y'=\left(\frac{1}{x+(1+x^2)^{1/2}}\right)+\left(\frac{x}{\left(x+(1+x^2)^{1/2}\right)\left(1+x^2\right)^{1/2}}\right)$$ $$y'=\left(\frac{1}{x+(1+x^2)^{1/2}}\right)+\left(\frac{x}{\left(x(1+x^2)^{1/2}\right)+(1+x^2)^{1}}\right)$$

At this point I can see that if I simplify further by adding the fractions I'll still have too many terms, and it will get awfully messy. The answer per the book (below) has far fewer terms than mine. I'd just like to know where I've gone wrong in my algebra. Thank you for your help. Here's the correct answer:

$$y'=\frac{1}{\sqrt{1+x^2}}$$

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  • $\begingroup$ I notice right away that an extra power appears as you use the natural log rule. Why is the $x$ squared? $\endgroup$ – The Count Dec 26 '16 at 23:59
  • $\begingroup$ Umm... The problem seems to ask for $\sqrt{1 + x}$, not $\sqrt{1 + x^2}$ $\endgroup$ – David Bowman Dec 27 '16 at 0:00
  • $\begingroup$ Ah thanks, my title and given problem is incorrect. Editing now $\endgroup$ – baverso Dec 27 '16 at 0:01
  • $\begingroup$ I find I make fewer mistakes with problems like these if I start by exponentiating both sides. Just an idea. $\endgroup$ – GFauxPas Dec 27 '16 at 0:25
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The function is $y=\ln(x+\sqrt{1+x^2})$, so $$ y'=\frac{1}{x+\sqrt{1+x^2}}\left(1+\frac{x}{\sqrt{1+x^2}}\right)= \frac{1}{x+\sqrt{1+x^2}}\frac{\sqrt{1+x^2}+x}{\sqrt{1+x^2}}= \frac{1}{\sqrt{1+x^2}} $$

I don't think your result is wrong per se: you just miss to notice some simplifications. Avoid distributing before factoring.

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    $\begingroup$ So you combined the $1+x/\sqrt{1+x^2}$ into one fraction, and then noticed that a factor canceled out. Since OP didn't do this, he didn't notice the factor that cancels. $\endgroup$ – Akiva Weinberger Dec 27 '16 at 0:03
  • $\begingroup$ @AkivaWeinberger Yes, the OP performed the multiplication too early. $\endgroup$ – egreg Dec 27 '16 at 0:06
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Hint: Your function is the hyberbolic arcsin (or the inverse hyperbolic sin). In order to derivate such function one should use the theorem for inverse derivatives. That is $$(f^{-1}(x))'=\frac{1}{f^{'}(f^{-1}(x))}.$$ See the link for more information . https://en.m.wikipedia.org/wiki/Inverse_hyperbolic_function

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You were almost there. You just missed the calculation. You got this:

$$y'=\left(\frac{1}{x+(1+x^2)^{1/2}}\right)\cdot\left(1+\frac{x}{(1+x^2)^{1/2}}\right)$$

You should just work with the right term:

$$y'=\left(\frac{1}{x+(1+x^2)^{1/2}}\right)\cdot\left(\frac{(1+x^2)^{1/2}+x}{(1+x^2)^{1/2}}\right)=\left(\frac{(1+x^2)^{1/2}+x}{[(1+x^2)^{1/2}+x](1+x^2)^{1/2}}\right)$$

Now just simplify the term $(1+x^2)^{1/2}+x$ and get:

$$y'=\frac{1}{(1+x^2)^{1/2}}=\frac{1}{\sqrt{1+x^2}}$$

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Let $u=x+\sqrt{1+x^2}$. Then $$u'=1+\left[\frac{1}{2}(1+x^2)^{-\frac{1}{2}}\cdot 2x\right]=1+\frac{x}{\sqrt{1+x^2}}=\frac{\sqrt{1+x^2}+x}{\sqrt{1+x^2}}.$$ Thus, $$y'=\frac{1}{u}\cdot u'=\frac{1}{x+\sqrt{1+x^2}}\cdot \frac{\sqrt{1+x^2}+x}{\sqrt{1+x^2}}=\frac{1}{\sqrt{1+x^2}}.$$

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You wrote:

Distribute the left term across the two right terms

That is the same mistake that we see when we consider simplifying $\dfrac 3 4\times \dfrac 5 3.$ One can multiply: $$ \frac{3\times 5}{4\times 3} = \frac{15}{12} $$ but that is not a good way to simplify. Usually one should cancel before multiplying:$\require{cancel}$ $$ \frac{\bcancel 3} 4 \times \frac 5 {\bcancel 3}. $$ Do the same thing here: cancel before multiplying: \begin{align} \left(\frac{1}{x+(1+x^2)^{1/2}}\right)\cdot\left(1+\frac{x}{(1+x^2)^{1/2}}\right) & = \left(\frac{1}{x+(1+x^2)^{1/2}}\right)\cdot\left( \frac{ (1+x^2)^{1/2} + x}{(1+x^2)^{1/2}} \right) \\[10pt] & =\left(\frac 1 {\cancel{x+(1+x^2)^{1/2}}}\right)\cdot\left( \frac{ \cancel{ (1+x^2)^{1/2} + x}}{(1+x^2)^{1/2}} \right) \end{align}

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