解：?$(1)$?原式?$=\frac {3×(\sqrt {10}-\sqrt 7)}{(\sqrt {10}+\sqrt 7)(\sqrt {10}-\sqrt 7)}=\sqrt {10}-\sqrt 7$?
原式?$=\frac {10-7}{\sqrt {10}+\sqrt 7}=\frac {(\sqrt {10})^2-(\sqrt 7)^2}{\sqrt {10}+\sqrt 7}=\frac {(\sqrt {10}+\sqrt 7)(\sqrt {10}-\sqrt 7)}{\sqrt {10}+\sqrt 7}=\sqrt {10}-\sqrt 7$?
?$(2) $?原式?$ =\frac {\sqrt 3-1}{(\sqrt 3+1)(\sqrt 3-1)}+\frac {\sqrt 5-\sqrt 3}{(\sqrt 5+\sqrt 3)(\sqrt 5-\sqrt 3)}+·s+\frac {\sqrt {2 \mathrm n+1}-\sqrt {2 \mathrm n-1}}{(\sqrt {2 \mathrm n+1}+\sqrt {2 \mathrm n-1})(\sqrt {2 \mathrm n+1}-\sqrt {2 \mathrm n-1})}$?
?$ =\frac {\sqrt 3-1}2+\frac {\sqrt 5-\sqrt 3}2+·s+\frac {\sqrt {2 \mathrm n+1}-\sqrt {2 \mathrm n-1}}2$?
?$ =\frac {\sqrt {2 \mathrm n+1}-1}2 $?
