数学题：有一个池塘，池塘里的荷叶覆盖面积每周扩大两倍，照这样计算9周的时候，荷叶将覆盖整个池塘。那么第（　）周的时候，荷叶覆盖面积正好占池塘面积的四分之一。

数学题解答：这题用倒推法解决。当第九周荷叶长满池塘的时候，可以用1来表示。那么第八周就是1/2，第七周就是1/4。总之，前一周荷叶的面积总是后一周的一半。从1开始往前推，很快就找到1/4了。

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