A= $\frac{2}{\sqrt{7}-5}$-$\frac{2}{\sqrt{7}+5}$
= $\frac{2(\sqrt{7}+5)}{(\sqrt{7})^2-5^2}$-$\frac{2(\sqrt{7}-5)}{(\sqrt{7})^2-5^2}$
= $\frac{2\sqrt{7}+10}{7-25}$-$\frac{2\sqrt{7}-10}{7-25}$
= $\frac{2\sqrt{7}+10-2\sqrt{7}+10}{-18}$
= $\frac{20}{-18}$
= $\frac{-10}{9}$
B= $\frac{\sqrt{7}+\sqrt{5}}{\sqrt{7}-\sqrt{5}}$+$\frac{\sqrt{7}-\sqrt{5}}{\sqrt{7}+\sqrt{5}}$
= $\frac{(\sqrt{7}+\sqrt{5})(\sqrt{7}+\sqrt{5})}{(\sqrt{7})^2-(\sqrt{5})^2}$+$\frac{(\sqrt{7}-\sqrt{5})(\sqrt{7}-\sqrt{5})}{(\sqrt{7})^2-(\sqrt{5})^2}$
= $\frac{7+\sqrt{35}+\sqrt{35}+5}{7-5}$+$\frac{7-\sqrt{35}-\sqrt{35}+5}{7-5}$
= $\frac{12+2\sqrt{35}}{2}$+$\frac{12-2\sqrt{35}}{2}$
= $\frac{12+2\sqrt{35}+12-2\sqrt{35}}{2}$
= $\frac{24}{2}$
= 12
#NOCOPY
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