Consider the equation
\[ y ^ { \prime \prime } ( x ) + P ( x ) y ^ { \prime } ( x ) + Q ( x ) y ( x ) = 0\tag{35} \]
and show that if \(y _ { 1 }\) is a solution to this equation, then also \(\begin{array} { r } { y _ { 2 } = y _ { 1 } \int ^ { x } d s \frac { e ^ { - \int ^ { s } d t P ( t ) } } { [ y _ { 1 } ( s ) ] ^ { 2 } } } \end{array}\) is.