Đáp án:
$\displaystyle \begin{array}{{>{\displaystyle}l}} a.\ x=2005\\ b.\ x=3\sqrt{2} ;\ x=\sqrt{2}\\ c.\ x=\frac{1\pm \sqrt{13}}{2} \end{array}$
Giải thích các bước giải:
$\displaystyle \begin{array}{{>{\displaystyle}l}} a.\ \Leftrightarrow \frac{x}{2} =\frac{2005}{2}\\ \Leftrightarrow x=2005\\ b.\ \Leftrightarrow \left( x-3\sqrt{2}\right)\left( x+\sqrt{2}\right) =0\\ \Leftrightarrow x=3\sqrt{2} ;\ x=\sqrt{2}\\ c.\ \Leftrightarrow x^{2}( x+3)^{2} +9x^{2} =7( x+3)^{2}\\ \Leftrightarrow x^{2}\left( x^{2} +6x+9\right) +9x^{2} =7.\left( x^{2} +6x+9\right)\\ \Leftrightarrow x^{4} +6x^{3} +11x^{2} -42x-63=0\\ \Leftrightarrow \left( x^{2} -x-3\right)\left( x^{2} +7x+21\right) =0\\ \Leftrightarrow x^{2} -x-3=0\\ \Leftrightarrow \frac{1\pm \sqrt{13}}{2} \end{array}$
