Áp dụng công thức : $\sqrt{A.B}=\sqrt{A}.\sqrt{B}$ , ta có :

a.  $\sqrt{7}.\sqrt{63}=\sqrt{7.63}=\sqrt{7^{2}.3^{2}}=\sqrt{7^{2}}.\sqrt{3^{2}}=7.3=21$

b.  $\sqrt{2,5}.\sqrt{30}.\sqrt{48}=\sqrt{2,5.30.48}=\sqrt{25.3.48}=\sqrt{60^{2}}=60$

c.  $\sqrt{0,4}.\sqrt{6,4}=\sqrt{0,4.6,4}=\sqrt{0,04.64}=\sqrt{0,04}.\sqrt{64}=0,2.8=1,6$

d.  $\sqrt{2,7}.\sqrt{5}.\sqrt{1,5}=\sqrt{2,7.5.1,5}=\sqrt{3^{2}.1,5^{2}}=\sqrt{3^{2}}.\sqrt{1,5^{2}}=3.1,5=4,5$