$\text{a)}0,81$  

Obliczamy błąd względny podanego przybliżenia liczby ¹⁷/₂₁:

$\frac{\left|\frac{17}{21}-0,81\right|}{\left|\frac{17}{21}\right|}=\frac{\left|\frac{17}{21}-\frac{81}{100}\right|}{\left|\frac{17}{21}\right|}=\frac{\left|\frac{1700}{2100}-\frac{1701}{2100}\right|}{\left|\frac{17}{21}\right|}=\frac{\left|-\frac{1}{2100}\right|}{\frac{17}{21}}=\frac{1}{{\sout{2100}}^{100}}\cdot \frac{{\sout{21}}^1}{17}=\frac{1}{100}\cdot \frac{1}{17}=\frac{1}{1700}$ $\frac{\left|\frac{17}{21}-0,81\right|}{\left|\frac{17}{21}\right|}=\frac{\left|\frac{17}{21}-\frac{81}{100}\right|}{\left|\frac{17}{21}\right|}=\frac{\left|\frac{1700}{2100}-\frac{1701}{2100}\right|}{\left|\frac{17}{21}\right|}=\frac{\left|-\frac{1}{2100}\right|}{\frac{17}{21}}=\frac{1}{{\sout{2100}}^{100}}\cdot \frac{{\sout{21}}^1}{17}=\frac{1}{100}\cdot \frac{1}{17}=\frac{1}{1700}$    

  

$\text{b)}0,8$

$\frac{\left|\frac{17}{21}-0,8\right|}{\frac{17}{21}}=\frac{\left|\frac{17}{21}-\frac{8}{10}\right|}{\frac{17}{21}}=\frac{\left|\frac{170}{210}-\frac{168}{210}\right|}{\frac{17}{21}}=\frac{\left|\frac{2}{210}\right|}{\frac{17}{21}}=\frac{\frac{2}{210}}{\frac{17}{21}}=\frac{2}{{\sout{210}}^{10}}\cdot \frac{{\sout{21}}^1}{17}=\frac{2}{10}\cdot \frac{1}{17}=\frac{2}{170}$

 

$\text{c)}0,8096$

$\frac{\left|\frac{17}{21}-0,8096\right|}{\frac{17}{21}}=\frac{\left|\frac{17}{21}-\frac{8096}{10000}\right|}{\frac{17}{21}}=\frac{\left|\frac{170000}{210000}-\frac{170016}{210000}\right|}{\frac{17}{21}}=\frac{\left|-\frac{16}{210000}\right|}{\frac{17}{21}}=\frac{16}{{\sout{210000}}^{10000}}\cdot \frac{{\sout{21}}^1}{17}=\frac{16}{10000}\cdot \frac{1}{17}=\frac{16}{170000}$