\[x² - 9x - 17 = 0\]
\[x_{1} + x_{2} = 9;\]
\[x_{1} \cdot x_{2} = - 17\]
\[x_{1}\ и\ \ x_{2} \Longrightarrow корни\ уравнения.\]
\[x_{1}^{2} + x_{2}^{2} =\]
\[= \left( x_{1}^{2} + 2x_{1}x_{2} + x_{2}^{2} \right) - 2x_{1}x_{2} =\]
\[= \left( x_{1} + x_{2} \right)^{2} - 2x_{1}x_{2} =\]
\[= 81 - 2 \cdot ( - 17) = 81 + 34 =\]
\[= 115\]