将一块带有尖刃的石器和一根强壮的树枝接在一起，利用这根长矛人类可以极大地提高速度、力度、效率和安全性。
Attach a sharp stone to the end of astout piece of wood, and with this lever you can hugely increase thespeed, force, effectiveness and safety with which the axe can bewielded.
这条的规律可以简单描述为：如果一根树干分叉为两个树枝，那么两个树枝的横截面积之和等于树干的横截面积；如果树枝接着分别分叉为两根小树枝，那么这四个次生分枝的横截面积总和仍等于树干的横截面积……依此类推。
If those two branches in turn each split into two branches, the area of the cross sections of the four additional branches together will equal the area of the cross section of the trunk. And so on.
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