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【简答题】
阅读以下的材料： 如果两个正数a，b，即a>0，b>0，有下面的不等式： 当且仅当a=b时取到等号，我们把 叫做正数的算术平均数，把 叫做正数a，b的几何平均数，于是上述不等式可表述为：两个正数的算术平均数不小于（即大于或等于）它们的几何平均数。它在数学中有广泛的应用，是解决最值问题的有力工具。下面举一例子： 例：已知x>0，求函数 的最小值。 解：令a=x， ，则有 ，得 ，当且仅当 时，即x=2时，函数有最小值，最小值为2。 根据上面回答下列问题： ①已知x>0，则当x=______时，函数 取到最小值，最小值为______； ②用篱笆围一个面积为100m 2 的矩形花园，问这个矩形的长、宽各为多少时，所用的篱笆最短，最短的篱笆周长是多少； ③已知x>0，则自变量x取，函数 取到最大值，最大值为多少？
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constantly decreasing as engine load increase
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