# Finding the power series of $\arcsin x$

I’m trying to find the power series of $\arcsin x$.

This is what I did so far:
$(\arcsin x)'=\frac{1}{\sqrt{1-x^2}}$, so $\arcsin x=\int \sqrt{\sum_{n=0}^{\infty}x^{2n}}$.

(for $|x|<1$)

Any hints for what should I do further?

Thanks a lot.

#### Solutions Collecting From Web of "Finding the power series of $\arcsin x$"

The generalized binomial thorem states that

$$(1+x)^{\alpha} = \sum_{k=0}^{\infty} {{\alpha\choose {k}} x^k}$$

The definition still holds

$${\alpha\choose {k}} =\frac{\alpha(\alpha-1)\cdots(\alpha-k+1)}{k!}$$

As people are suggesting, you need to see that

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

thus you can integrate the binomial series of

$$(1-x^2)^{-\frac{1}{2}} = \sum_{k=0}^{\infty} (-1)^k {{-\frac{1}{2}\choose {k}} x^{2k}}$$

Using the definition shows that

$${-\frac{1}{2}\choose {k}} = {( – 1)^k}\frac{{(2k – 1)!!}}{{k!{2^k}}} = {( – 1)^k}\frac{{\left( {2k – 1} \right)!!}}{{\left( {2k} \right)!!}}$$

Putting this in the sum produces:

$${\left( {1 – {x^2}} \right)^{-\frac{1}{2}}} = \sum\limits_{k = 0}^\infty {\frac{{\left( {2k – 1} \right)!!}}{{\left( {2k} \right)!!}}{x^{2k}}}$$

Finally this yields:

$${\sin ^{ – 1}}x = \sum\limits_{k = 0}^\infty {\frac{{\left( {2k – 1} \right)!!}}{{\left( {2k} \right)!!}}\frac{{{x^{2k + 1}}}}{{2k + 1}}}$$

Remember that by definition

$$0!! = 1$$
$$(-1)!! = 1$$

If you’re not comfortable with that simply put:

$${\sin ^{ – 1}}x = x + \sum\limits_{k = 1}^\infty {\frac{{\left( {2k – 1} \right)!!}}{{\left( {2k} \right)!!}}\frac{{{x^{2k + 1}}}}{{2k + 1}}}$$