这是一个临界点，考察它的二阶导数,会发现这确实是一个极大值点。

That is a critical point. And we used the second derivative to see that this critical point is a local maximum.

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因此，存在一个临界周期使高宽比限值取得极大值。

So there exists a critical period, at which, the limit ratio reaches its maximum value.

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因此，存在一个临界周期使高宽比限值取得极大值。

So there exists a critical period, at which the height width ratio reaches its maximum value.

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