angularity of tangent line 切线夹角
pseudo-tangent line [数] 伪切线 ; 拟切线
slope of the tangent line 切线的斜率
equation of tangent line 则其切线方程式
tangent line [数] 切线
rule of tangent line 切线规则
If the two level curves are tangent to each other that means they have the same tangent line. That means that the normal vectors should be parallel. Let me maybe draw a picture here.
如果两个等高线相切，说明它们的切线相同，也就是说他们的法向量应该平行，我再在这里画个图。
And visually what we're accomplishing is somehow to take the hyperbola, and take a point on the hyperbola, and figure out some tangent line.
从视觉上看，我们要做的是，想办法，在这个反函数，在反函数上取一个点，设法找出切线。
A derivative at a given point is just the slope of the tangent line that kisses that point.
一个给定点的导数不过就是一条切线的斜率轻吻了那个点。
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