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。
Equation about isometric polygon curve is set up with differential geometry theory. The curve is proved to be convex curve which the distance between any parallel tangent lines is equal.
应用微分几何理论，导出了等距曲线的方程，证明了它是具有任意两条平行切线之间距离都相等的卵形线。
From the rolling contact problem of two viscoelastic cylinders with parallel axis, a series of normal-tangent contact coupled integration equation is obtained.
从两个平行轴粘弹性圆柱体的接触问题出发，可以得到一个法向接触和切向接触耦合的积分方程组。
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