全微分 dz $$ \mathrm{d}z = \frac{\partial z}{\partial x} \Delta x + \frac{\partial z}{\partial x} \Delta y = \frac{\parti...

费马定理，$f(x)$在$x\_0$处可导，且在$x\_0$取得极值，必有$f'(x\_0)=0$ $x\_0$处取极大值的充分条件，$f^{(n)}(x\_0)<0$,n 为偶数 $x\_0$处取极小值的充分条件，$f^{(n)}(x...