# Finding $\int^{1}_{-1}\frac{x^3}{\sqrt{1-x^2}}\ln\bigg(\frac{1+x}{1-x}\bigg)dx$

Finding $$\displaystyle \int^{1}_{-1}\frac{x^3}{\sqrt{1-x^2}}\ln\bigg(\frac{1+x}{1-x}\bigg)dx$$

Try: Let $$\displaystyle I = \int^{1}_{-1}\frac{x^3}{\sqrt{1-x^2}}\ln\bigg(\frac{1+x}{1-x}\bigg)dx$$ (Function is even )

$$I = 2\int^{1}_{0}\bigg[\ln(1+x)-\ln(1-x)\bigg]\frac{x^3}{\sqrt{1-x^2}}dx$$

Using By parts

$$I = 2\bigg[\ln\bigg(\frac{1+x}{1-x}\bigg)\bigg(\sqrt{1-x^2}-\sqrt[3]{1-x^2}\bigg)\bigg]\bigg|^{1}_{0}+4\int^{1}_{0}\frac{1}{1-x^2}\cdot \bigg(\sqrt{1-x^2}-\sqrt[3]{1-x^2}\bigg)dx$$

Could some help me to solve second Integration, Thanks

• Maybe you need Beta function. – Szeto Sep 29 '18 at 9:35
• It is $$\frac{5\pi}{3}$$ – Dr. Sonnhard Graubner Sep 29 '18 at 9:51
• There's an error in your by-part turning the $\frac{x^3}{\sqrt{1-x^2}}dx$ into its anti-derivative – Lee David Chung Lin Sep 29 '18 at 10:05

Since $$\int \frac{x^3}{\sqrt{1-x^2}} \, dx = -\frac{1}{3}(2+x^2)\sqrt{1-x^2}$$, integration by parts yields
\begin{align*} \int_{-1}^{1} \frac{x^3}{\sqrt{1-x^2}} \log\bigg(\frac{1+x}{1-x}\bigg) \, dx &= \int_{-1}^{1} \frac{2(2+x^2)}{3\sqrt{1-x^2}} \, dx. \end{align*}
Now the substitution $$x = \sin\theta$$ quickly yields the answer $$\frac{5}{3}\pi$$.
Let us consider your integral: $$I = \int^{1}_{-1}\frac{x^3}{\sqrt{1-x^2}}\ln\bigg(\frac{1+x}{1-x}\bigg)dx$$ Separate the integral: $$I= \int^{1}_{-1}\frac{x^3}{\sqrt{1-x^2}}\ln (1+x)\ dx-\int^{1}_{-1}\frac{x^3}{\sqrt{1-x^2}}\ln (1-x)\ dx$$ Consider the right-hand integral. Let $$x=-t$$ such that $$dx=-dt$$, where $$t\in (1,-1)$$: $$I= \int^{1}_{-1}\frac{x^3}{\sqrt{1-x^2}}\ln (1+x)\ dx-\int^{-1}_{1}\frac{(-t)^3}{\sqrt{1-(-t)^2}}\ln (1-(-t))\ (-dt)$$ Incorporate a minus sign into the integral in order to switch to interval to $$t\in (-1,1)$$: $$I= \int^{1}_{-1}\frac{x^3}{\sqrt{1-x^2}}\ln (1+x)\ dx+\int^{1}_{-1}\frac{t^3}{\sqrt{1-t^2}}\ln (1+t)\ dt$$ Recognize that $$t$$ is a dummy variable. Let $$t=x$$ such that $$dt=dx$$, where $$x\in (-1,1)$$: $$I= \int^{1}_{-1}\frac{x^3}{\sqrt{1-x^2}}\ln (1+x)\ dx+\int^{1}_{-1}\frac{x^3}{\sqrt{1-x^2}}\ln (1+x)\ dx=2\int^{1}_{-1}\frac{x^3}{\sqrt{1-x^2}}\ln (1+x)\ dx$$ Apply integration by parts: $$I=-\frac{2}{3}\sqrt{1-x^2}(x^2+2)\ln(1+x)\bigg|^{1}_{-1}+\frac{2}{3}\int^{1}_{-1}\sqrt{1-x^2}\frac{x^2+2}{1+x}\ dx=\frac{2}{3}\int^{1}_{-1}\sqrt{1-x^2}\frac{x^2+2}{1+x}\ dx$$ Let $$x=\sin(u)$$ such that $$dx=\cos(u)\ du$$, where $$u\in (-\pi/2,\pi/2]$$: $$I=\frac{2}{3}\int^{\pi/2}_{-\pi/2}\cos^2(u)\frac{\sin^2(u)+2}{1+\sin(u)}\ du=\frac{2}{3}\int^{\pi/2}_{-\pi/2}(\sin^2(u)+2)\frac{1-\sin^2(u)}{1+\sin(u)}\ du$$ Perform long-division on the integrand: $$I=\frac{2}{3}\int^{\pi/2}_{-\pi/2}(\sin^2(u)+2)(1-\sin(u))\ du=\frac{2}{3}\int^{\pi/2}_{-\pi/2}\sin^2(u)\ du-\frac{2}{3}\int^{\pi/2}_{-\pi/2}\sin^3(u)\ du+\frac{4}{3}\int^{\pi/2}_{-\pi/2}du-\frac{4}{3}\int^{\pi/2}_{-\pi/2}\sin(u)\ du$$ Since $$\sin^3(u)$$ and $$\sin(u)$$ are odd, their integrals evaluate to zero. Rewrite $$\sin^2(u)$$ by making use of a double-angle identity: $$I=\frac{1}{3}\int^{\pi/2}_{-\pi/2}(1-\cos(2u))\ du+\frac{4}{3}\int^{\pi/2}_{-\pi/2}du=\frac{5}{3}u-\frac{1}{6}\sin(2u)\bigg|^{1}_{-1}$$ Thus, $$I=\frac{5}{3}\pi$$