I must calculate a following integral
$$\int \frac{dx}{x^{2}\sqrt{1+x^{2}}}$$
with a subsitution like this $x = \frac{1}{t}, t<0$
I'm on this step $$\int \frac{dt}{\frac{1}{t}\sqrt{t^{2} + 1}}$$
I don't know what I should do now... (or maybe it's wrong).