Đáp án:
a. \( - \dfrac{{14\sqrt 3 }}{{\sqrt 5 }}\)
Giải thích các bước giải:
\(\begin{array}{l}
a.\sqrt {\dfrac{3}{5}} + 3\sqrt {\dfrac{5}{3}} - 4\sqrt {15} \\
= \sqrt {\dfrac{3}{5}} + \sqrt {15} - 4\sqrt {15} \\
= \sqrt {\dfrac{3}{5}} - 3\sqrt {15} \\
= \dfrac{{\sqrt 3 - 3.5\sqrt 3 }}{{\sqrt 5 }}\\
= - \dfrac{{14\sqrt 3 }}{{\sqrt 5 }}\\
b.DK:a \ge 0\\
\sqrt {\dfrac{{3a}}{7}} - 2\sqrt {\dfrac{{7a}}{3}} + \sqrt {21a} \\
= \dfrac{{3\sqrt a - 2.7\sqrt a + 21\sqrt a }}{{\sqrt {21} }}\\
= \dfrac{{10\sqrt a }}{{\sqrt {21} }}\\
c.DK:y \ge 0\\
2\sqrt {\dfrac{{84}}{5}} + \sqrt {\dfrac{{45y}}{2}} - \sqrt {10} \\
= \dfrac{{2.2\sqrt {42} + 15\sqrt y - 10}}{{\sqrt {10} }}\\
= \dfrac{{4\sqrt {42} + 15\sqrt y - 10}}{{\sqrt {10} }}
\end{array}\)
