Lời giải:
a.
\(\sqrt{16a}+2\sqrt{40a}-3\sqrt{90a}=\sqrt{16}.\sqrt{a}+2\sqrt{40}.\sqrt{a}-3\sqrt{90}.\sqrt{a}\)
\(=\sqrt{a}(\sqrt{16}+2\sqrt{40}-3\sqrt{90})=\sqrt{a}(4-5\sqrt{10})\)
b.
\(=(\sqrt{28}-\sqrt{12}-\sqrt{7})\sqrt{7}+2\sqrt{21}\)
\(=\sqrt{28}.\sqrt{7}-\sqrt{12}.\sqrt{7}-7+2\sqrt{21}\)
\(=\sqrt{4}.\sqrt{7}.\sqrt{7}-\sqrt{4}.\sqrt{3}.\sqrt{7}-7+2\sqrt{21}\)
\(=2.7-2\sqrt{21}-7+2\sqrt{21}=14-7=7\)
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