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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(n+k)(n-k)=48
可能的情况有1*48 2*24 3*16 4*12 6*8
因为n+k+n-k=2n是个偶数 所以1*48和3*16被排除 剩下3租
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