$\displaystyle \begin{array}{{>{\displaystyle}l}} Đặt\ A=\sqrt{7+\sqrt{13}} +\sqrt{7-\sqrt{13}}\\ A^{2} =\left(\sqrt{7+\sqrt{13}} +\sqrt{7-\sqrt{13}}\right)^{2}\\ A^{2} =7+\sqrt{13} +2\sqrt{\left( 7+\sqrt{13}\right)\left( 7-\sqrt{13}\right)} +7-\sqrt{13}\\ A^{2} =14+2\sqrt{49-13}\\ A^{2} =14+2.6\\ A^{2} =26\ \\ \rightarrow A=\sqrt{26} \end{array}$