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a)
\sqrt{48}=\sqrt{4}\sqrt{4}\sqrt{6}=4\sqrt{6}
b)
\sqrt{\frac{4}{81}}=\frac{2}{9}
c)
\sqrt{5}+\sqrt{20}=\sqrt{5}+\sqrt{4}\sqrt{5}=\sqrt{5}+2\sqrt{5}=3\sqrt{5}
d)
\sqrt{5+20}=\sqrt{25}=5
e)
\sqrt{3}+\sqrt{3}=2\sqrt{3}
f)
\sqrt{3}\cdot\sqrt{3}=\left(\sqrt{3}\right)^2=\sqrt{9}=3
g)
\sqrt{3}\cdot\sqrt{12}=\sqrt{3}\cdot\sqrt{3}\cdot\sqrt{4}=2\left(\sqrt{3}\right)^2=2\sqrt{9}=2\cdot3=6
h)
\sqrt{3}+\sqrt{12}=\sqrt{3}+2\sqrt{3}=3\sqrt{3}