xy（dy/dx）=1-x²==> ydy = (1/x - x) *dx /** 两边乘以dx/x==> 1/2*dy² = d(lnx) - 1/2*dx² ==> dy² = 2d(lnx) - dx² 两边积分得：y² = 2lnx - x² + c 该式即为微分方程的通解...