$$- \frac{3}{x^{2} + 1}$$
6*x
---------
2
/ 2\
\1 + x /
$$\frac{6 x}{\left(x^{2} + 1\right)^{2}}$$
/ 2 \
| 4*x |
6*|1 - ------|
| 2|
\ 1 + x /
--------------
2
/ 2\
\1 + x /
$$\frac{1}{\left(x^{2} + 1\right)^{2}} \left(- \frac{24 x^{2}}{x^{2} + 1} + 6\right)$$