$e^2=d^2+f^2$

$f^2=d^2+e^2$

$d^2=e^2+f^2$

 $20,\ 30,\ 40$  

 $\sqrt{9},\ \sqrt{16},\ \sqrt{25}$  

 $\frac{2}{3},\ \frac{4}{5},\ \frac{6}{7}$  

 $\sqrt{3}+1,\ \sqrt{3}-1,\ \sqrt{8}$