Bonjour;
a)
[tex]\dfrac{1}{4}+\dfrac{3}{4}\times\dfrac{5}{2}=\dfrac{1}{4}+\dfrac{3\times5}{4\times2}=\dfrac{1}{4}+\dfrac{15}{4\times2}=\dfrac{1\times2}{4\times2}+\dfrac{15}{4\times2}\\\\\\=\dfrac{2}{8}+\dfrac{15}{8}=\dfrac{2+15}{8}=\dfrac{17}{8}\ .[/tex]
b)
[tex]2\sqrt{2}-\sqrt{8}=\sqrt{2^2}\sqrt{2}-\sqrt{8}\\\\\\=\sqrt{2^2\times2}-\sqrt{8}=\sqrt{4\times2}-\sqrt{8}=\sqrt{8}-\sqrt{8}=0\ .[/tex]
c)
[tex]-\dfrac{5\times10^7\times2\times10^{-2}}{35\times10^3}=-\dfrac{5\times2\times10^7\times10^{-2}}{35\times10^3}\\\\\\=-\dfrac{10\times10^7\times10^{-2}}{35\times10^3}=-\dfrac{10^{1+7-2}}{35\times10^3}=-\dfrac{10^{6}}{35\times10^3}\\\\\\=-\dfrac{10^{6-3}}{35}=-\dfrac{10^3}{35}=-\dfrac{1000}{35}=-\dfrac{200}{7}\ .[/tex]
d)
[tex]\dfrac{\sqrt{128}}{4}-3\sqrt{72}=\dfrac{\sqrt{128}}{\sqrt{4^2}}-3\sqrt{36\times2}=\dfrac{\sqrt{128}}{\sqrt{4^2}}-3\sqrt{36\times2}\\\\\\=\sqrt{\dfrac{128}{4^2}}-3\sqrt{6^2\times2}=\sqrt{\dfrac{128}{16}}-3\sqrt{6^2}\sqrt2=\sqrt{8}-3\times6\sqrt{2}\\\\\\=2\sqrt{2}-18\sqrt{2}=-16\sqrt{2}\ .[/tex]