What is the equation to solve this problem?
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[tex]\displaystyle\bf\\x^2+x^2=10^2\\\\2x^2=19^2\\\\x^2=\frac{10^2}{2}\\\\ \sqrt{x^2}=\sqrt{\frac{10^2}{2}}\\\\x=\frac{10\sqrt{2}}{\sqrt{2}\times\sqrt{2}}\\\\x=\frac{10\sqrt{2}}{2}\\\\\boxed{\bf x=5\sqrt{2}~~~~~~Correct~ansver:~~D)}[/tex]