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I am having a problem solving an integral. I am stuck in an infinite loop. Integral is:

$$\int{\frac{dx}{\sqrt{1-x^2}\arcsin{x}}}$$

I have separated it in dv and u on this way:

$$u = \frac{dx}{\sqrt{1-x^2}}$$ $$dv = \frac{1}{\arcsin{x}}$$

And the using:

$$u v - \int{v \, du}$$

I get again:

$$\int{\frac{dx}{\sqrt{1-x^2}\arcsin{x}}}$$

I dont know, but probably, I am doing something wrong. I am new at solving Integrals so I am learning :) According to my book the result should be:

$$\ln({\arcsin{x}})-C$$

And it will be true if I didn't had $$\sqrt{1-x^2}$$ but on this way I have no idea.

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Hint. Use substitution. If $u=\arcsin x$, what is $du$? –  Arturo Magidin Jun 24 '12 at 1:26
    
Thank you :) Is there any way to know if i have to use Integration by Parts or u-substitution ? –  Andres Jun 24 '12 at 1:32
2  
There are no algorithms (that's why integration is harder than differentiation). But here, experience will let you recognize $\frac{1}{\sqrt{1-x^2}}$ as the derivative of $\arcsin x$, and if you notice that the only thing left over is an expression of $\arcsin x$, then that should suggest trying a substitution. –  Arturo Magidin Jun 24 '12 at 1:36
    
You set $dv = 1/arcsin(x)$? How did you even integrate that? –  Eugene Jun 24 '12 at 6:06
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3 Answers

up vote 5 down vote accepted

Do not use Integration by Parts. Use $u$-substitution. Let $u=\sin^{-1}(x)$. Then $du=dx/\sqrt{1-x^2}$

So now your integral is $$\int \frac{du}{u}$$

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Note: Your comment at the end is incorrect. It is not true that $$\int\frac{dx}{\arcsin x} = \ln(\arcsin x)+ C\tag{Wrong!}$$

This is, unfortunately, a common mistake. Don't fall for it again!

While $$\int\frac{dx}{x} = \ln|x|+C$$ is true, in general, $$\int\frac{dx}{f(x)}$$ is not equal to $\ln|f(x)|+C$. If you differentiate $\ln|f(x)|$, you'll notice that you get $\frac{f'(x)}{f(x)}$. It is precisely because you have a $\frac{1}{\sqrt{1-x^2}}$ that you do get the natural logarithm of an arcsine.

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Thank you for the advice :) –  Andres Jun 24 '12 at 1:48
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"Another" method, which in fact is like substitution but, with some practice and care, perhaps is a little faster. Since $\,\displaystyle{(\arcsin x)'=\frac{1}{\sqrt{1-x^2}}}\,$ , and assuming we know $\,\displaystyle{\int\frac{f'}{f}\,dx=\log|f|+C}\,$, we can write

$$\int\frac{dx}{\sqrt{1-x^2}\arcsin x}\,dx=\int \frac{1}{\arcsin x}\,\frac{1}{\sqrt{1-x^2}}\,dx=$$$$=\int \frac{1}{\arcsin x}\,d(\arcsin x)=\log|\arcsin x|+C$$

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