0\\x^2=\frac{4}{5-x^2}\end{array}\right.\\\left\lbrace\begin{array}{l} x>0\\x^2(5-x^2)=4\end{array}\right.\\\left\lbrace\begin{array}{l} x>0\\x^4-5x^2+4=0\end{array}\right.\\\left\lbrace\begin{array}{l} x>0\\(x^2-1)(x^2-4)=0\end{array}\right.\\ \left[\begin{array}{l}x=1\\x=2 \end{array}\right." alt="2\lg x-\lg 4=-\lg(5-x^2)\\2\lg x=\lg4-\lg(5-x^2)\\ \left\lbrace\begin{array}{l} x>0\\x^2=\frac{4}{5-x^2}\end{array}\right.\\\left\lbrace\begin{array}{l} x>0\\x^2(5-x^2)=4\end{array}\right.\\\left\lbrace\begin{array}{l} x>0\\x^4-5x^2+4=0\end{array}\right.\\\left\lbrace\begin{array}{l} x>0\\(x^2-1)(x^2-4)=0\end{array}\right.\\ \left[\begin{array}{l}x=1\\x=2 \end{array}\right." align="absmiddle" class="latex-formula">