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$$\frac{\D x_j}{\D t}=f_j(x_1,\cdots,x_n)$$   $$\mbox{($j=1,2,\cdots,n$)}$$

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$$\frac{\D x}{\D t}=x^2,\quad x(0)=1$$

$$x(t)=\frac{1}{1-t}$$   $$\mbox{($t\in(-\infty,1)$)}$$$$.$$

$$\frac{\D x}{\Dt}=x^2+p(t)x+q(t)$$

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