Rút gọn các biểu thức:
a) \(\sqrt 8 .\sqrt {18} :\frac{{\sqrt 5 }}{{\sqrt 2 }}\)
b) \(\sqrt {75} :\sqrt {45} .\frac{3}{{\sqrt {10} }}\)
Dựa vào: \(\frac{{\sqrt a }}{{\sqrt b }} = \frac{{\sqrt a .\sqrt b }}{{{{\left( {\sqrt b } \right)}^2}}} = \frac{{\sqrt {ab} }}{b}(a \ge 0,b > 0)\)
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\(\sqrt {\frac{a}{b}} = \sqrt {\frac{{ab}}{{{b^2}}}} = \frac{{\sqrt {ab} }}{b}(a \ge 0,b > 0)\)
a) \(\sqrt 8 .\sqrt {18} :\frac{{\sqrt 5 }}{{\sqrt 2 }} \)
\(= 2\sqrt 2 .3\sqrt 2 .\frac{{\sqrt 2 }}{{\sqrt 5 }} = \frac{{12\sqrt 2 .\sqrt 5 }}{{\sqrt 5 .\sqrt 5 }} = \frac{{12\sqrt {10} }}{5}\).
b) \(\sqrt {75} :\sqrt {45} .\frac{3}{{\sqrt {10} }} \)
\(= \frac{{\sqrt {75} }}{{\sqrt {45} }}.\frac{3}{{\sqrt {10} }} = \frac{{5\sqrt 3 }}{{3\sqrt 5 }}.\frac{3}{{\sqrt 5 .\sqrt 2 }} = \frac{{\sqrt 3 }}{{\sqrt 2 }} = \frac{{\sqrt 6 }}{2}\).