найдите производную
[m]y`=\frac{(6x)`\cdot (1+x^2)-(6x)\cdot (1+x^2)`}{(1+x^2)^2}=\frac{6\cdot (1+x^2)-(6x)\cdot (2x)}{(1+x^2)^2}=\frac{6+6x^2-12x^2}{(1+x^2)^2}=\frac{6-6x^2}{(1+x^2)^2}[/m]
