A discussion of the relations between continuity, partial derivative, directional derivative and differentiability of binary function is helpful for us to study binary function.
探讨二元函数的连续性、偏导数、方向导数以及可微性之间的关系，有助于我们对二元函数的学习。
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The Relation between the Continuity, the Partial Derivatives, the Directional Derivatives and the Differentiability of a Function.