http://projecteuler.net/problem=28 [Answer:669171001]
2011-11-29 23:37
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/* 43 44 45 46 47 48 49 42 21 22 23 24 25 26 41 20 7 8 9 10 27 40 19 6 1 2 11 28 39 18 5 4 3 12 29 38 17 16 15 14 13 30 37 36 35 34 33 32 31 right-up 1, 9, 25, 49 -> n*n left-up 1, 7, 21, 43 -> n*n-n+1 left-down 1, 5, 17, 37 -> n*n-2*n+2 right-down 1, 3, 13, 31 -> n*n-3*n+3 sum 4, 24,76, 160 -> 4*n*n-6*n+6 n=2*k-1 4*n*n-12*n+12=16kk-16k+4-12k+6+6=16*k*k-28*k+16 sum=8*k*(k+1)*(2*k+1)/3-14*k*(k+1)+16*k-3 // -3 for three 1 */ #include <iostream> using namespace std; int main() { int n = 1001; int k = (n + 1) / 2; int sum = 8*k*(k+1)*(2*k+1)/3 - 14*k*(k+1) + 16*k - 3; cout << sum << endl; return 0; }
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