Solution
= Solution
Write
$$
y=y_0+\Delta y,
\qquad
y_0=\frac{\sin\phi}{b},
$$
where $\Delta y$ is first order in $M$. Substitution into $y''+y=3My^2$ and omission of terms of order $M\Delta y$ and $(\Delta y)^2$ gives
$$
\boxed{
\Delta y''+\Delta y
=3M y_0^2
=\frac{3M}{2b^2}(1-\cos2\phi).}
$$
This is the linearized orbit equation used in the <perturbative derivation of Schwarzschild light deflection>.