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题库 / GMATAdvanced-PS-14
Let n and k be positive integers with k ≤ n. From an n × n array of dots, a k × k array of dots is selected.The figure above shows two examples where the selected k × k array is enclosed in a square. How many pairs (n, k) are possible so that exactly 48 of the dots in the n × n array are NOT in the selected k × k array?
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再停下来多想两秒钟。。。。。n*n - k*k = 48
(n - k) * (n + k) = 48
n = 7, k = 1
n = 13, k = 11
n = 8, k = 4
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