\[\boxed{\text{660\ (660).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[a^{2} + b^{2} = 34;\ \ ab = 15;\ \ \]
\[a > 0;\ \ b > 0\]
\[(a + b)^{2} = a^{2} + 2\text{ab} + b^{2} =\]
\[= 34 + 2 \cdot 15 = 34 + 30 = 64\]
\[тогда\ a + b = 8.\]
\[Ответ:8.\]