\[y^{2} - 15y - 13 = 0\ \]
\[D = b^{2} - 4ac =\]
\[= 225 - 4 \cdot ( - 13) =\]
\[= 225 + 52 > 0\]
\[y_{1} \cdot y_{2} = - 13 \Longrightarrow корни\ разных\ \]
\[знаков.\]