Giải thích các bước giải:
Ta có:
\(\begin{array}{l}
A = \dfrac{{1 + {{\cos }^2}x}}{{1 - {{\cos }^2}x}} - 2{\cot ^2}x\\
= \dfrac{{1 + {{\cos }^2}x}}{{{{\sin }^2}x}} - 2.\dfrac{{{{\cos }^2}x}}{{{{\sin }^2}x}}\,\,\,\,\,\,\,\,\left( {{{\sin }^2}x + {{\cos }^2}x = 1} \right)\\
= \dfrac{{1 + {{\cos }^2}x - 2{{\cos }^2}x}}{{{{\sin }^2}x}}\\
= \dfrac{{1 - {{\cos }^2}x}}{{{{\sin }^2}x}}\\
= \dfrac{{{{\sin }^2}x}}{{{{\sin }^2}x}} = 1
\end{array}\)
