\[y_{p}(t)=y_{2}(t) \int_{t_{1}}^{t} \frac{f(\tau) y_{1}(\tau)}{a(\tau) W(\tau)} d \tau-y_{1}(t) \int_{t_{0}}^{t} \frac{f(\tau) y_{2}(\tau)}{a(\tau) W(\tau)} d \tau .\label{eq:6}\