# Trouble solving $\int\sqrt{1-x^2} \, dx$

I am trying to learn how to solve integrals and I've got the hang out of a lot of examples, but I haven't got the slightest idea how to solve this example, this is how far I've got:

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

Thank you

• See this. Dec 24 '13 at 16:32
• $\sin^2t+\cos^2t=1\iff\cos t=\sqrt{1-\sin^2t}$ . Let then $x=\sin t$ . Dec 24 '13 at 16:45

Using Trigonometric substitution, $\arcsin x=\phi$

$\implies (i)x=\sin\phi$ and

$(ii)-\frac\pi2\le \phi\le\frac\pi2$ based on principal value of inverse sine ratio,

$\implies\cos\phi\ge0$

$$\int\sqrt{1-x^2}dx=\int|\cos\phi|\cos\phi d\phi = \int\cos^2\phi d\phi$$

$$= \frac12\int(1+\cos2\phi) d\phi$$

$$=\frac12\left(\phi+\frac{\sin2\phi}2\right)+K=\frac{\phi+\sin\phi\cos\phi}2+K$$

• @Mark, my pleasure. We can use $15$ of this(sosmath.com/tables/integral/integ10/integ10.html) setting $n=\frac12$ and use $\int\frac{dx}{\sqrt{1-x^2}}=\arcsin x$ Dec 24 '13 at 16:57
• Thank you a lot for this list! I definitely think it will be of great use, sure I will try, because the Wolfram|Alpha result contained $\arcsin(x)$
– Mark
Dec 24 '13 at 17:03

$\newcommand{\cos}{\sin}$ Let's try $$\int_0^{x}\sqrt{1-a^2}\text da$$ You can generalize from that. Now, $\sqrt{1-x^2}$ describes a semicircle. Consider the following image:

So we have $\int_0^x\sqrt{1-a^2}\text da=A_1+A_2$. $A_2$ is a right triangle with base $x$ and height $\sqrt{1-x^2},$ so $A_2=\frac12x\sqrt{1-x^2}$. Meanwhile, $A_2$ is a sector of the circle. Note that the angle $$\theta=\angle (0,1)O(x,\sqrt{1-x^2})=\frac{\sin^{-1}x}1=\sin^{-1}x$$ Since the angle from the center is proportional to the area, $A_2=\frac{\theta}{2\pi}\cdot \pi r^2=\frac{\sin^{-1}x}{2}$

Thus $$\int_0^x\sqrt{1-a^2}\text da=\frac{\sin^{-1}x+x\sqrt{1-x^2}}{2}$$

• A very descriptive answer, thank you. This is of big help to me trying to learn the integrals
– Mark
Dec 24 '13 at 17:28
• You typed $\int \limits_0^{\color{red}x}f(\color{red}x)\mathrm d\color{red}x$ throughout. Jan 10 '14 at 20:54
• I'm an idiot.${}$ Jan 10 '14 at 23:48

subsititute $x =sin\theta$, this gives, $dx = cos\theta d\theta$

hence, $\int \sqrt{(1-x^2)} dx = \int (\sqrt{1 - sin^2\theta} ). cos\theta d\theta = \int cos^2 \theta d\theta$

Remember that, $cos(\alpha + \beta) = cos \alpha .cos \beta - sin \alpha . sin \beta$, substituting, $\alpha = \beta = \theta$,

1. $(cos^2\theta - sin^2\theta) = cos(2\theta)$, as well

2. $(cos^2\theta + sin^2\theta) = 1$

Adding equation 1. and 2. yields,

$2 cos^2 \theta = 1 + cos2\theta ~ \implies ~ cos^2\theta = \frac{1}{2}(1+cos2\theta)$

hence going back to integral, $\int cos^2\theta d\theta = \frac{1}{2}\int (1+cos2\theta) d\theta = \frac{1}{2}(\theta + \frac{sin2\theta}{2}) + C$

By reverse substitution, $\theta = cos^{-1}x = arccos(x)$ and $sin2\theta = 2.sin\theta . cos\theta$

The solution of indefinite integral is ,

$\int \sqrt{(1-x^2)} dx = \frac{1}{2} ( \theta + \frac{2.sin\theta.cost\theta}{2} )+ C \implies \frac{1}{2}(\theta + sin\theta. cos\theta) + C$

$\int \sqrt{(1-x^2)} dx \implies \frac{1}{2}(arccos(x) + x\sqrt{1-x^2}) + C$

I, personally, suggest @lab's way because it is general and suitable for students, but you may use this strange substitution as well: $$1-x^2=x^2t^2$$

• Is this substitution useful in other types of integrals that are more complicated? Maybe it's just because I've never seen it written this way before.
– Joe
Dec 24 '13 at 17:38
• @Joe: See here, but lab's way is better. Since this integral is very easy. Dec 24 '13 at 18:00

Your idea of integrating by parts is good (but you did it wrong):

\begin{align} \int\sqrt{1 - x^2} \, dx &= x\sqrt{1-x^2} - \int x\frac{-2x}{2\sqrt{1-x^2}} \, dx \\ &= x\sqrt{1-x^2} + \int\frac{x^2}{\sqrt{1-x^2}}\,dx\\ &= x\sqrt{1-x^2} + \int\frac{x^2-1+1}{\sqrt{1-x^2}}\,dx\\ &= x\sqrt{1-x^2} - \int\sqrt{1-x^2}\,dx+\int\frac{1}{\sqrt{1-x^2}}\,dx\\ &= x\sqrt{1-x^2} + \arcsin x - \int\sqrt{1-x^2}\,dx \end{align} Therefore $$2\int\sqrt{1 - x^2} \, dx = x\sqrt{1-x^2} + \arcsin x$$ and $$\int\sqrt{1 - x^2} \, dx = \frac{1}{2}x\sqrt{1-x^2} + \frac{1}{2}\arcsin x + c$$