21 lines
423 B
C++
21 lines
423 B
C++
#include<iostream>
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#include<cstdio>
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#include<cstring>
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using namespace std;
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int n,num[1010];
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int main(){
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int t;
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scanf("%d",&t);
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while(t--){
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scanf("%d",&n);
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int ans=0;
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for(int i=0;i<n;i++){
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scanf("%d",&num[i]);
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ans+=1*num[i]*(n-1-num[i]);
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}
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ans/=2;
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printf("%.3f\n",1.0-ans*1.0/((n*(n-1)*(n-2))/6));
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}
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return 0;
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}
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