Oh, a function of two variables.
这的确是一个二元函数。
Well, let's say how to visualize a function of two variables.
如何想象一个有两个变量的函数。
That was our first example of a function of two variables possibly.
这是二元函数中的第一个例子。
So, when we think of a graph, really, it is a function of two variables.
因此当我们在想像一个图像时，其实那只是一个二元函数的图像。
OK, so now, let's say that we had, actually, a function of two variables.
可能是这样的，现在我们可以说，有一个含有两个变量的函数。
So, we said when we have a function of two variables, we have the gradient vector.
我们说，当一个二元函数存在时，就有梯度向量。
Limit of function of two variables is more complex than that of one variable.
二元函数的极限远比一元函数的极限复杂，但它们之间又有密切的联系。
we've seen is how to actually view a function of two variables in terms of its graph and its contour plot.
我们学过怎样认识二元函数,从它的图像和等高线的角度来看。
I should say that is for a function of two variables to try to decide whether a given critical point is a minimum, a maximum or a saddle point.
但这种办法并不普遍，只能用于二元函数，来判断是极小值点、极大值点还是鞍点。
I should say that is for a function of two variables to try to decide whether a given critical point is a minimum, a maximum or a saddle point.
但这种办法并不普遍，只能用于二元函数，来判断是极小值点、极大值点还是鞍点。
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