a) \(\sqrt 3 \times \sqrt {75} = \sqrt {3 \times 75} = \sqrt {225} = 15\)
b)
\(\eqalign{& \sqrt {20} \times \sqrt {72} \times \sqrt { {4,9} } = \sqrt {\left( {20 \times 72 \times 4,9} \right)} = \sqrt {20 \times 72 \times 10 \times 4,9} \cr & = \sqrt {{7056} } = \sqrt {{{{84 }^2}} } = 84 \cr} \)
