Is 5^k less than 1000?
1. 5^(k + 1) > 3000
2. 5^(k - 1) = 5^k - 500
答案是B,怎么做的?
5^4=625,5^5肯定大于1000;
5^(k+1)>3000推出5^k>600,k>=4都可以满足,但就不能判断是否小于1000了;
5^(k-1)=5^k-500,
(1-1/5)5^k=500
5^k=500*5/4=5^4<1000
所以B
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