$\text{a)}2\sqrt{2}\approx 2\cdot 1,4\approx 1,8$     

$\text{b)}\sqrt{2}\cdot \sqrt{7}\approx 1,4\cdot 2,6=\frac{14}{10}\cdot \frac{26}{10}=\frac{7}{5}\cdot \frac{13}{5}=\frac{91}{25}=3\frac{16}{25}=3\frac{64}{100}=3,64\approx 3,6$  

$\text{c)}\sqrt[3]{2}-\sqrt{7}\approx 1,3-2,6=-1,3$  

$\text{d)}\sqrt{7}:\sqrt{2}\approx 2,6:1,4=\frac{26}{10}:\frac{14}{10}=\frac{26}{10}\cdot \frac{10}{14}=\frac{26}{14}=\frac{13}{12}=1\frac{1}{12}=1,08\left(3\right)\approx 1,1$