MHB Integral of sqrt(1+x^2)/x: Help Solving w/ Trig Substitution

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The integral of sqrt(1+x^2)/x can be solved using trigonometric substitution by setting x = tan(θ), leading to the integral transforming into a form involving secant and cosecant functions. The substitution simplifies the integral to a combination of terms that can be integrated separately. An alternative approach using hyperbolic functions, specifically x = sinh(t), is also discussed, yielding similar results through different identities and substitutions. Ultimately, both methods lead to the final result of the integral being expressed in terms of x, including logarithmic and square root components. The discussion highlights the versatility of trigonometric and hyperbolic substitutions in solving integrals.
MarkFL
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Here is the question:

Integral of sqrt(1+x^2)/x using trigonometric substitution?


Hi, I keep getting the answer wrong on this problem, and I was wondering if someone could please help me figure out how to solve it? I understand the basic concept, and I know that x=tan(θ), but I'm having trouble figuring out what trig identities to use. Thank you for your help!

I have posted a link there to this question so the OP can view my work.
 
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Hello KTRavenclaw,

We are given to integrate:

$$\int\frac{\sqrt{1+x^2}}{x}\,dx$$

I agree with your choice of substitution:

$$x=\tan(\theta)\,\therefore\,dx= \sec^2(\theta)\,d\theta$$

Using the Pythagorean identity $$\tan^2(u)+1=\sec^2(u)$$ we obtain:

$$\int\frac{\sec^3(\theta)}{\tan(\theta)}\, d\theta=\int\csc(\theta)\sec^2(\theta) \,d\theta$$

Using the above mentioned Pythagorean identity, we may write:

$$\int\csc(\theta)\left(\tan^2(\theta)+1 \right) \,d\theta=\int \sec(\theta)\tan(\theta)+\frac{1}{\sin(\theta)}\, d\theta$$

Now, using the following:

$$\frac{d}{du}\left(\sec(u) \right)=\sec(u)\tan(u)$$

$$\sin(2u)=2\sin(u)\cos(u)$$

We may write the integral as:

$$\int\,d\left(\sec(\theta) \right)+\frac{1}{2}\int\frac{1}{ \sin\left(\frac{\theta}{2} \right) \cos\left(\frac{\theta}{2} \right)}$$

If we multiply the second integrand by $$1=\frac{\sec^2\left(\frac{\theta}{2} \right)}{\sec^2\left(\frac{\theta}{2} \right)}$$ and put the constant with the numerator of the integrand, we obtain:

$$\int\,d\left(\sec(\theta) \right)+\int\frac{\dfrac{1}{2}\sec^2\left( \frac{\theta}{2} \right)}{\tan\left(\frac{\theta}{2} \right)}\, d\theta$$

Now, for the second integral, apply the substitution:

$$u=\tan\left(\frac{\theta}{2} \right)\,\therefore\,du=\frac{1}{2}\sec^2\left( \frac{\theta}{2} \right)\, d\theta$$

Thus, we now have:

$$\int\,d\left(\sec(\theta) \right)+\int\frac{1}{u}\,du$$

And so using the rules of integration, we obtain:

$$\sec(\theta)+\ln|u|+C$$

Back-substitute for $u$:

$$\sec(\theta)+\ln\left|\tan\left(\frac{\theta}{2} \right) \right|+C$$

Using the half-angle identity for tangent:

$$\tan\left(\frac{u}{2} \right)=\csc(u)-\cot(u)$$ we have:

$$\sec(\theta)+\ln\left|\csc(\theta)-\cot(\theta) \right|+C$$

Back-substitute for $\theta$:

$$\sqrt{1+x^2}+\ln\left|\frac{\sqrt{1+x^2}-1}{x} \right|+C$$

And so we may conclude:

$$\int\frac{\sqrt{1+x^2}}{x}\,dx= \sqrt{1+x^2}+\ln\left|\frac{\sqrt{1+x^2}-1}{x} \right|+C$$
 
MarkFL said:
Using the Pythagorean identity $$\tan^2(u)+1=\sec^2(u)$$ we obtain:

$$\int\frac{\sec^3(\theta)}{\tan(\theta)}\, d\theta=\int\csc(\theta)\sec^2(\theta) \,d\theta$$

I'm not sure how you make this step. Is this just a simple trig identity?
 
tmt said:
I'm not sure how you make this step. Is this just a simple trig identity?

Think of the tangent function like this:

$$\tan(u)=\frac{\sin(u)}{\cos(u)}=\sin(u)\sec(u)$$
 
In my opinion a more straightforward substitution is $\displaystyle \begin{align*} x = \sinh{(t)} \implies \mathrm{d}x = \cosh{(t)}\,\mathrm{d}t \end{align*}$, giving

$\displaystyle \begin{align*} \int{ \frac{\sqrt{1 + x^2}}{x}\,\mathrm{d}x} &= \int{ \frac{\sqrt{1 + \sinh^2{(t)}}}{\sinh{(t)}}\,\cosh{(t)}\,\mathrm{d}t } \\ &= \int{ \frac{\cosh^2{(t)}}{\sinh{(t)}}\,\mathrm{d}t } \\ &= \int{ \frac{1 + \sinh^2{(t)}}{\sinh{(t)}}\,\mathrm{d}t } \\ &= \int{ \left[ \frac{1}{\sinh{(t)}} + \sinh{(t)} \right] \,\mathrm{d}t } \\ &= \int{ \frac{\sinh{(t)}}{\sinh^2{(t)}}\,\mathrm{d}t } + \int{ \sinh{(t)}\,\mathrm{d}t } \\ &= \int{ \frac{\sinh{(t)}}{\cosh^2{(t)} - 1} \,\mathrm{d}t } + \cosh{(t)} + C_1 \\ &= \int{ \frac{\sinh{(t)}}{\left[ \cosh{(t)} - 1 \right] \left[ \cosh{(t)} + 1 \right] } \,\mathrm{d}t } + \cosh{(t)} + C_1 \end{align*}$

You can solve that resulting integral with $\displaystyle \begin{align*} u = \cosh{(t)} \implies \mathrm{d}u = \sinh{(t)} \,\mathrm{d}t \end{align*}$ giving $\displaystyle \begin{align*} \int{\frac{1}{\left( u - 1 \right) \left( u + 1 \right) } \,\mathrm{d}u } + \cosh{(t)} + C_1 \end{align*}$.

Apply Partial Fractions

$\displaystyle \begin{align*} \frac{A}{u - 1} + \frac{B}{u + 1} &\equiv \frac{1}{ \left( u - 1 \right) \left( u + 1 \right) } \\ A\,\left( u + 1 \right) + B \,\left( u - 1 \right) &\equiv 1 \end{align*}$

Let $\displaystyle \begin{align*} u = -1 \end{align*}$ to find $\displaystyle \begin{align*} -2\,A = 1 \implies A = -\frac{1}{2} \end{align*}$.

Let $\displaystyle \begin{align*} u = 1 \end{align*}$ to find $\displaystyle \begin{align*} 2\,B = 1 \implies B = \frac{1}{2} \end{align*}$. Then the integral is

$\displaystyle \begin{align*} \int{ \left[ -\frac{1}{2}\,\left( \frac{1}{u - 1} \right) + \frac{1}{2}\,\left( \frac{1}{u + 1} \right) \right] \,\mathrm{d}u } + \cosh{(t)} + C_1 &= \frac{1}{2} \int{ \left( \frac{1}{u + 1} - \frac{1}{u - 1} \right) \,\mathrm{d}u } + \cosh{(t)} + C_1 \\ &= \frac{1}{2} \,\left( \ln{ \left| u + 1 \right| } - \ln{ \left| u - 1 \right| } \right) + C_2 + \cosh{(t)} + C_1 \\ &= \frac{1}{2} \ln{ \left| \frac{u + 1}{u - 1} \right| } + \cosh{(t)} + C \textrm{ where } C = C_2 + C_1 \\ &= \frac{1}{2}\ln{ \left| \frac{\cosh{(t)} + 1}{\cosh{(t)} - 1 } \right| } + \sqrt{ 1 + \sinh^2{(t)} } + C \\ &= \frac{1}{2}\ln{ \left| \frac{\left[ \cosh{(t)} + 1 \right] ^2}{\left[ \cosh{(t)} - 1 \right] \left[ \cosh{(t)} + 1 \right] } \right| } + \sqrt{1 + x^2} + C \\ &= \frac{1}{2}\ln{ \left| \frac{\cosh^2{(t)} + 2\cosh{(t)} + 1}{\cosh^2{(t)} - 1 } \right| } + \sqrt{1 + x^2} + C \\ &= \frac{1}{2}\ln{ \left| \frac{1 + \sinh^2{(t)} + 2\,\sqrt{ 1 + \sinh^2{(t)}} + 1}{\sinh^2{(t)}} \right| } + \sqrt{1 + x^2} + C \\ &= \frac{1}{2}\ln{ \left| \frac{2 + x^2 + 2\,\sqrt{1 + x^2}}{x^2} \right| } + \sqrt{1 + x^2} + C \end{align*}$
 
\displaystyle\int \frac{\sqrt{1+x^2}}{x}\,dx
\text{Let }\,x = \tan\theta,\; dx = \sec^2\!\theta\,d\theta

\text{Substitute: }\;\int \frac{\sec\theta}{\tan\theta}\,\sec^2\theta\,d\theta \;=\;\int\frac{\sec^3\theta}{\tan\theta}\,d\theta \;=\;\int\frac{1}{\cos^3\theta}\,\frac{\cos\theta}{\sin\theta}\,d\theta \;=\;\int\frac{d\theta}{\sin\theta\cos^2\theta}

. . =\;\int \frac{\sec^2\theta}{\sin\theta}\,d\theta \;=\; <br /> \int \frac{1}{\sin\theta} (\tan^2\theta +1)\,d\theta \;=\; \int\left(\frac{\sin\theta}{\cos^2\theta} + \frac{1}{\sin\theta}\right)\,d\theta

. . =\;\int (\sec\theta\tan\theta + \csc\theta)\,d\theta \;=\; \sec\theta + \ln|\csc\theta - \cot\theta| + C\text{Back-substitute: }\;\sqrt{1+x^2} + \ln\left|\frac{\sqrt{1+x^2} - 1}{x}\right| + C
 
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