在几何课上，我们学习了如何画一条曲线的切线。

In geometry class, we learned how to draw a tangent to a curve.

想像一下，如果从人类首次在地球上出现开始画一条曲线，追踪在此之后人类社会的经济发展轨迹。

Imagine a chart that begins when man first appeared on the planet and tracks the economic growth of societies from then forward.