What you're looking for is their identity is \(\displaystyle{{\csc}^{{2}}{x}}-{1}={{\cot}^{{2}}{x}}\).You can derive this by starting from \(\displaystyle{{\sin}^{{2}}{x}}+{{\cos}^{{2}}{x}}={1}\): \(\displaystyle{{\sin}^{{2}}{x}}+{{\cos}^{{2}}{x}}={1}\) \(\displaystyle\cancel{{\frac{{{\sin}^{{2}}{x}}}{{{\sin}^{{2}}{x}}}}}^{{{1}}}+{\stackrel{{{{\cot}^{{2}}{x}}}}{\overbrace{{\frac{{{\cos}^{{2}}{x}}}{{{\sin}^{{2}}{x}}}}}}}={\stackrel{{{{\csc}^{{2}}{x}}}}{\overbrace{{\frac{{1}}{{{\sin}^{{2}}{x}}}}}}}\) \(\displaystyle{\mathbf{{{1}+{{\cot}^{{2}}{x}}={{\csc}^{{2}}{x}}}}}\) Thus: \(\displaystyle{\color{blue}{\frac{{{\cot}^{{2}}{x}}}{{{\csc{{x}}}+{1}}}}}\) \(\displaystyle=\frac{{{{\csc}^{{2}}{x}}-{1}}}{{{\csc{{x}}}+{1}}}\) \(\displaystyle=\frac{{{\left({\csc{{x}}}-{1}\right)}\cancel{{{\left({\csc{{x}}}+{1}\right)}}}}}{\cancel{{{\left({\csc{{x}}}+{1}\right)}}}}\) \(\displaystyle={\color{blue}{{\csc{{x}}}-{1}}}\) ...if and only if \(\displaystyle{\csc{{x}}}+{1}\ne{0}\). If \(\displaystyle{\csc{{x}}}+{1}={0}\), then \(\displaystyle\frac{{1}}{{\sin{{x}}}}=-{1}\), which is the case when \(\displaystyle{x}=\frac{{{3}\pi}}{{2}}\pm{2}{n}\pi\) for all \(\displaystyle{n}\in\mathbb{Z}\). Hence, this answer is valid when \(\displaystyle{x}\ne\frac{{{3}\pi}}{{2}}\pm{2}{n}\pi\) for all \(\displaystyle{n}\in\mathbb{Z}\)."" What you're looking for is their identity is \(\displaystyle{{\csc}^{{2}}{x}}-{1}={{\cot}^{{2}}{x}}\). You can derive this by starting from \(\displaystyle{{\sin}^{{2}}{x}}+{{\cos}^{{2}}{x}}={1}\): \(\displaystyle{{\sin}^{{2}}{x}}+{{\cos}^{{2}}{x}}={1}\) \(\displaystyle\cancel{{\frac{{{\sin}^{{2}}{x}}}{{{\sin}^{{2}}{x}}}}}^{{{1}}}+{\stackrel{{{{\cot}^{{2}}{x}}}}{\overbrace{{\frac{{{\cos}^{{2}}{x}}}{{{\sin}^{{2}}{x}}}}}}}={\stackrel{{{{\csc}^{{2}}{x}}}}{\overbrace{{\frac{{1}}{{{\sin}^{{2}}{x}}}}}}}\) \(\displaystyle{\mathbf{{{1}+{{\cot}^{{2}}{x}}={{\csc}^{{2}}{x}}}}}\) Thus: \(\displaystyle{\color{blue}{\frac{{{\cot}^{{2}}{x}}}{{{\csc{{x}}}+{1}}}}}\) \(\displaystyle=\frac{{{{\csc}^{{2}}{x}}-{1}}}{{{\csc{{x}}}+{1}}}\) \(\displaystyle=\frac{{{\left({\csc{{x}}}-{1}\right)}\cancel{{{\left({\csc{{x}}}+{1}\right)}}}}}{\cancel{{{\left({\csc{{x}}}+{1}\right)}}}}\) \(\displaystyle={\color{blue}{{\csc{{x}}}-{1}}}\) ...if and only if \(\displaystyle{\csc{{x}}}+{1}\ne{0}\). If \(\displaystyle{\csc{{x}}}+{1}={0}\), then \(\displaystyle\frac{{1}}{{\sin{{x}}}}=-{1}\), which is the case when \(\displaystyle{x}=\frac{{{3}\pi}}{{2}}\pm{2}{n}\pi\) for all \(\displaystyle{n}\in\mathbb{Z}\). Hence, this answer is valid when \(\displaystyle{x}\ne\frac{{{3}\pi}}{{2}}\pm{2}{n}\pi\) for all \(\displaystyle{n}\in\mathbb{Z}\)