\(x^2-19x-56=0\)
\(\Leftrightarrow x^2-19x=56\)
\(\Leftrightarrow x^2-19x+\frac{361}{4}=56+\frac{361}{4}\)
\(\Leftrightarrow x^2-2.x.\frac{19}{2}+\left(\frac{19}{2}\right)^2=\frac{585}{4}\)
\(\Leftrightarrow\left(x-\frac{19}{2}\right)^2=\frac{585}{4}\)
\(\Leftrightarrow\left(x-\frac{19}{2}\right)^2=\left(\pm\sqrt{\frac{585}{4}}\right)^2\)
\(\Leftrightarrow\left[{}\begin{matrix}x-\frac{19}{2}=\sqrt{\frac{585}{4}}\\x-\frac{19}{2}=-\sqrt{\frac{585}{4}}\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}x-\frac{19}{2}=\frac{\sqrt{585}}{2}\\x-\frac{19}{2}=-\frac{\sqrt{585}}{2}\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}x=\frac{\sqrt{585}+19}{2}\\x=\frac{-\sqrt{585}+19}{2}\end{matrix}\right.\)
1; D. 0
