Can the following integral integrated by parts?
$$\int\ln x\;\sin^{-1} x\, \operatorname d\!x$$
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$\begingroup$ Can you please explain how to do that? $\endgroup$– Swapnanil SahaAug 14, 2012 at 17:22
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1$\begingroup$ @swapnanilsaha There should be an empty check mark next to answers to your questions. Clicking that will accept someones answer. Note that you can only accept one answer. $\endgroup$– Holdsworth88Aug 14, 2012 at 17:24
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1$\begingroup$ @Holdsworth88 : Thankyou very much. $\endgroup$– Swapnanil SahaAug 14, 2012 at 17:26
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1$\begingroup$ "Can the following integral integrated by parts?" - repeated use of it, yes. To start you out: $u=\arcsin\,x$, $\mathrm dv=\log\,x\mathrm dx$. $\endgroup$– J. M. ain't a mathematicianAug 14, 2012 at 17:27
3 Answers
Integration by parts formula
$$\int u(x)\underset{dv}{\underbrace{v^{\prime }(x)dx}}=u(x)v(x)-\int v(x) \underset{du}{\underbrace{u^{\prime }(x)dx}}.$$
If we apply the LIATE rule to
$$\int \ln x\arcsin x\,dx,$$
we should choose
$$u =\ln x,\quad dv=\arcsin x\,dx.$$
Normally there is a choice of the terms $u,v'$ of the integrand $uv'$ that makes integration much easier. However in the present case the other possible choice $u=\arcsin x,v'=\ln x$ leads to integrals of similar difficulties.
$$\begin{eqnarray*} I &=&\int \ln x\arcsin x\,dx, \qquad \text{(}v =\int \arcsin x\,dx\quad \text{see evaluation below)} \\ &=&\left( \ln x\right) \left( x\arcsin x+\sqrt{1-x^{2}}\right) -\int \frac{ 1}{x}\left( x\arcsin x+\sqrt{1-x^{2}}\right) \,dx \\ &=&\left( \ln x\right) \left( x\arcsin x+\sqrt{1-x^{2}}\right) -\int \arcsin x\,dx-\int \frac{1}{x}\sqrt{1-x^{2}}\,dx \\ &&\text{(see evaluation of the last integral below)} \\ &=&\left( \ln x\right) \left( x\arcsin x+\sqrt{1-x^{2}}\right) -\left( \sqrt{1-x^{2}}+x\arcsin x\right) \\ &&-\sqrt{1-x^{2}}-\frac{1}{2}\ln \frac{\sqrt{1-x^{2}}-1}{\sqrt{1-x^{2}}+1}+C. \\ &=&\left( -1+\ln x\right) \left( x\arcsin x+\sqrt{1-x^{2}}\right) -\sqrt{ 1-x^{2}} -\frac{1}{2}\ln \frac{\sqrt{1-x^{2}}-1}{\sqrt{1-x^{2}}+1}+C. \end{eqnarray*}$$
- Evaluation of $\int \arcsin x\,dx$ by substitution $$\begin{eqnarray*} \int \arcsin x\,dx &=&\int u\cos u\,du,\quad u=\arcsin x \\ &=&\cos u+u\sin u \\ &=&\sqrt{1-x^{2}}+x\arcsin x+C. \end{eqnarray*}$$
- Evaluation of $\int \frac{1}{x}\sqrt{1-x^{2}}\,dx$ by substitution and partial fractions $$\begin{eqnarray*} \int \frac{1}{x}\sqrt{1-x^{2}}\,dx &=&-\int \frac{u^{2}}{1-u^{2}}\,du,\qquad u=\sqrt{1-x^{2}} \\ &=&u+\frac{1}{2}\ln \left( u-1\right) -\frac{1}{2}\ln \left( u+1\right) \\ &=&u+\frac{1}{2}\ln \frac{u-1}{u+1} \\ &=&\sqrt{1-x^{2}}+\frac{1}{2}\ln \frac{\sqrt{1-x^{2}}-1}{\sqrt{1-x^{2}}+1}+C. \end{eqnarray*}$$
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1$\begingroup$ Alternatively, $$\frac{\sqrt{1-x^2}}{x}=\frac{1}{x\sqrt{1-x^2}}-\frac{x}{\sqrt{1-x^2}}$$ $\endgroup$ Aug 14, 2012 at 23:48
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$\begingroup$ @J.M. Thanks, with this split the integral becomes $$\int \frac{1}{x}\sqrt{1-x^{2}}\,dx=\sqrt{1-x^{2}}-\text{arctanh}\frac{1}{ \sqrt{1-x^{2}}}+C.$$ $\endgroup$ Aug 14, 2012 at 23:54
Following J.M.'s hint, you may want to prove, again integrating by parts, that $$\int\log x\,dx=x\log x-x+C\,\,,\,\,C=\,\,\text{a constant}$$and perhaps also to note that $$\int \frac{x}{\sqrt{1-x^2}}dx=-\frac{1}{2}\int\frac{-2x}{\sqrt{1-x^2}}dx$$
Let I = ∫ ln x * sin−1 x dx,
= x(ln x - 1) * arcsin x - ∫ x(ln x - 1) * 1/√(1 – x^2) dx
= x(ln x - 1) * arcsin x - ∫(ln x - 1) * x / √(1 – x^2) dx
= x(ln x - 1) * arcsin x + (ln x - 1) * √(1 – x^2) – ∫ (1/x) * √(1 – x^2) dx
Call this last integral I1.
Put u = x^2; then du/dx = 2x
∴ I1 = ∫ (1/(2u)) * √(1 – u) * du/dx dx
Put u = 1 – v^2, v ≥ 0; so du/dv = -2v
∴ I1 = ½ ∫ 1/(1 – v^2) * (-2v^2) dv
v^2 / (v^2 – 1) = (v^2 – 1 + 1) / (v^2 – 1) = 1 + 1 / (v^2 – 1)
= 1 + ½ (1/(v – 1) - 1/(v + 1))
So I1 = v + ½ ln (A * (v – 1) / (v + 1)), where A is the arbitrary constant
= √(1 – u) + ½ ln (A * (√(1 – u) – 1) / (√(1 – u) + 1)),
= √(1 – x^2) + ½ ln (A * (√(1 – x^2) – 1) / (√(1 – x^2) + 1)),
Add this to the other parts, and you are done.
I at first thought it could not be done in elementary functions; but there we are!)