在我们开始之前，因为我们需要的是导数，取方程两边的导数。

Before we do that, since what we're after is the derivative, let's take the derivative of both sides.

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记住，当写的是近似值时，两边并不完全相等，但如果取很小的变化时，那么，最终两边的值将越来越接近。

Remember when we write approximately equal that means it is not quite the same, but if we take smaller variations then actually we will end up with values that are closer and closer.

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该模型认为，当主干与侧枝的截面积之间存在类似平行四边形法则的关系时，分枝的体积取极小值。

This model suggests that, when the relation "parallelogram rule" exists between main stem and sub branches, the total volume of branches is minimal.

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