17. Find the value (in the simplest form) of $\sqrt{9+4 \sqrt{5}}-\sqrt{9-4 \sqrt{5}}$.
17 Ans: 4 .
Completing the squares, one sees that the expression $=\sqrt{(2+\sqrt{5})^{2}}-\sqrt{(2-\sqrt{5})^{2}}=$ 4.