\[ \begin{split} &\lim_{n \to \infty} \frac{1-2+3-4+...-2n}{\sqrt{n^2+1}}=\\[16pt] &=\lim_{n \to \infty} \frac{\left(1+3+...+(2n-1)\right)-(2+4+...+2n)}{\sqrt{n^2+1}}=\\[16pt] &\{\text{w liczniku mamy dwie sumy ciągów arytmetycznych}\}\\[16pt] &=\lim_{n \to \infty} \frac{\dfrac{(2n-1)+1}{2}\cdot n-\dfrac{2n+2}{2}\cdot n}{\sqrt{n^2+1}}=\\[16pt] &=\lim_{n \to \infty} \frac{n^2-n^2-n}{\sqrt{n^2+1}}=\\[16pt] &=\lim_{n \to \infty} \frac{-n}{\sqrt{n^2+1}}\frac{:n}{:n}=\\[16pt] &=\lim_{n \to \infty} \dfrac{-\dfrac{n}{n}}{\sqrt{\dfrac{n^2}{n^2}+\dfrac{1}{n^2}}}=\\[16pt] &=\lim_{n \to \infty} \dfrac{-1}{\sqrt{1+\dfrac{1}{n^2}}}=\\[16pt] &=\frac{-1}{\sqrt{1}}=-1 \end{split} \]