解：??$(1) \sqrt {4+\frac 4{15}}=4 \sqrt {\frac 4{15}} $??
驗(yàn)證：??$ \sqrt {4+\frac 4{15}}=\sqrt {\frac {64}{15}}=\sqrt {\frac {4^2 ×4}{15}}=4 \sqrt {\frac 4{15}}$??
??$ (2) \sqrt {a+\frac a{a^2-1}}=a \sqrt {\frac a{a^2-1}} $??
證明：??$ \sqrt {a+\frac a{a^2-1}}=\sqrt {\frac {a^3}{a^2-1}}=a \sqrt {\frac a{a^2-1}} $??
