Beautiful Indefinite Integrals. [closed]

These are some of the integrals with beautiful solutions I came across-

$$\int \frac{x^2}{(x\sin x+\cos x)^2} dx$$

$$\int\frac {1}{\sin^3x+\cos^3x} dx$$

$$\int \frac{1}{x^4+1}dx$$

I'd love if you share some of the ones you came across.

Solution 1:

\begin{align} I_1 & = \int \sqrt{ \sqrt{ x + 2\sqrt{2x-4} } + \sqrt{ x - 2\sqrt{2x-4} } } \,\mathrm{d}x \, , \quad x>4\\ I_2 & = \int \log( \log x) + \frac{2}{\log x} - \frac{1}{(\log x)^2} \mathrm{d}x \\ I_4 & = \int (1 + 2x^2) e^{x^2}\, \mathrm{d}x \\ I_5 & = \int \frac{\sqrt{x+\sqrt{x^2+1\,}\,}\,}{\sqrt{x^2+1\,}\,} \mathrm{d}x \\ I_6 & = \int \frac{2^x 3^x}{9^x - 4^x} \,\mathrm{d}x \end{align} \begin{align*} I_7 = \int \left( \frac{\arctan x}{x - \arctan x}\right)^2 \mathrm{d}x = \frac{1 + x \arctan x}{\arctan x - x} = \frac{1}{\tan (\beta - \tan \beta)}\,, \end{align*} where $x = \tan \tan \beta$ or $\beta = \arctan (\arctan x)$. $$ I_6 = \int \frac{x^2+2x+1+ (3x+1)\sqrt{x+\ln x}}{x\,\sqrt{x+\ln x}(x+\sqrt{x+\ln x})}\mathrm{d}x = 2 (\sqrt{x+\ln x} + \ln(x+\sqrt{x+\ln x})) + C $$ I have a bunch more of these here, see p.68 for instance. (click on the problems for solution)

Solution 2:

This isn't indefinite. But it's crazy

$$ \int_0^{\pi/2} \frac{ d \theta}{\sqrt{a^2\cos^2\theta +b^2 \sin^2\theta }} = \frac{\pi}{2AGM(a,b)} $$

Where AGM is the arithmetic geometric mean.

Solution 3:

$$\int\dfrac{x^{4n}(1+x^{4n})}{1+x^2}dx$$

Why? Because from $0$ to $1$ they give good approximations of $\pi$. See this