等比数列

的各项均为正数,前四项之积等于64,那么

的最小值等于
。
解答:因为等比数列

的各项均为正数,由基本不等式可知


,当且仅当

时等号成立。由等比数列的性质,

,所以
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,所以
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,即


。
故答案为16
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