设y1, y2, y3都是微分方程y"+P(x)y'+Q(x)y=f(x)的解,且(y1-y2)/设y1, y2, y3都是微分方程y"+P(x)y'+Q(x)y=f(x)的解,且(y1-y2)/(y2-y3)≠常数,则该微分方程的通解为???此为选择题,答案是y=C1y1+C2y2+(1-C1-C2)y3是怎么解法?