[tex]\frac{a}{\sqrt{2}} =\frac{\sqrt{8}}{b}=>ab=\sqrt{2\times8} =\sqrt{16} =4[/tex]
[tex]\frac{\sqrt{3c} }{\sqrt{15} } =\frac{1}{\sqrt{15d}} =>\sqrt{45cd} =\sqrt{15} ()^2=>45cd=15=> cd=\frac{15}{45}=\frac{1}{3}[/tex]
[tex]=> ab-cd=4 - \frac{1}{3} =\frac{12}{3}-\frac{1}{3} =\frac{11}{3}[/tex]
