?$解：(2)①原式=\frac {2(\sqrt{5}－\sqrt{3})}{(\sqrt{5}+\sqrt{3})(\sqrt{5}－\sqrt{3})}$?
?$=\sqrt{5}－\sqrt{3}$?
?$②原式=\frac {5－3}{\sqrt{5}+\sqrt{3}}$?
?$=\frac {(\sqrt{5})2－(\sqrt{3})2}{\sqrt{5}+\sqrt{3}}$?
?$=\frac {(\sqrt{5}+\sqrt{3})(\sqrt{5}－\sqrt{3})}{\sqrt{5}+\sqrt{3}}$?
?$=\sqrt{5}－\sqrt{3}$?
?$(3)原式=\frac{1}{2}(\sqrt{3}-1+\sqrt{5}-\sqrt{3}+\sqrt{7}-\sqrt{5}+...+\sqrt{2n+1}-\sqrt{2n-1})$?
?$=\frac{1}{2}(\sqrt{2n+1}-1)$?
