Integrate by parts using $\int u\, v'\, dx = u v - \int u'\, v\, dx$:
Let $u = \tan^{-1} x \implies u' = \frac{1}{1 + x^2}$
Let $v' = x \implies v = \frac{x^2}{2}$
$$\int x \tan^{-1} x\, dx = \frac{x^2}{2}\tan^{-1} x - \int \frac{x^2}{2(1 + x^2)}\, dx$$
$$= \frac{x^2}{2}\tan^{-1} x - \frac{1}{2}\int \frac{(x^2 + 1) - 1}{1 + x^2}\, dx$$
$$= \frac{x^2}{2}\tan^{-1} x - \frac{1}{2}\int \left(1 - \frac{1}{1 + x^2}\right)\, dx$$
$$= \frac{x^2}{2}\tan^{-1} x - \frac{1}{2}x + \frac{1}{2}\tan^{-1} x + C$$
Hence, Option 4 is correct.
खंडशः समाकलन (Integration by parts) विधि द्वारा:
$u = \tan^{-1} x$ तथा $dv = x\, dx \implies v = \frac{x^2}{2}$:
$$\int x \tan^{-1} x\, dx = \frac{x^2}{2}\tan^{-1} x - \frac{1}{2}\int \frac{x^2}{1 + x^2}\, dx$$
$$= \frac{x^2}{2}\tan^{-1} x - \frac{1}{2}\int \left(1 - \frac{1}{1 + x^2}\right)\, dx$$
$$= \frac{x^2}{2}\tan^{-1} x - \frac{1}{2}x + \frac{1}{2}\tan^{-1} x + C$$
अतः विकल्प 4 सही उत्तर है।