求PREP模考的数学题，大牛快来

amy2098 · 发表于 2011-6-22 19:10:18

For a finite sequence of nonzero numbers, the number of variations in sign is defined as the number of pairs of consecutive terms of the sequence for which the product of the two consecutive terms is negative.What is the number of variations in sign for sequence 1,-3,2,5,-4,-6
A.1 B.2 C.3 D.4 E.5

If each term in the sum a1+a2+...+an is either 7 or 77,and the sum equals 350,which is n
A.38 B.39 C.40 D.41 D.42

labixiaoxinyue · 发表于 2011-6-22 19:32:16

这两题都不是难题呀！我没有记错的话，第一题答案是C，第二题答案也是C。
第一题就是说在这个数列中连续两数的乘积要是负数，那么1，-3是一对，-3,2是一对，还有就是5，-4是一对。那就是3！
第二题是说这个数列里面只有7和77.那设7有x个，77有y个。就是7x+77y=350。x,y都是正整数。然后去试一下，发现y=1,x=39满足，所以n=40!

求PREP模考的数学题，大牛快来

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