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。
2
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 paralleltangent 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。
3
From the rolling contact problem of two viscoelastic cylinders with parallel axis, a series of normal-tangent contact coupled integration equation is obtained.