2016多校联合训练第二场 1001 Acperience 公式推导

        简单的公式推导题，先把所有的数取绝对值，然后猜测k为wi的平均数，然后推导分子分母的公式，最后求分子分母的最大公因数后上下同除就好了，代码如下：

#pragma comment(linker, "/STACK:1024000000,1024000000")
#include<iostream>
#include<cstdlib>
#include<cstdio>
#include<cmath>
#include<cstring>
#include<vector>
#include<map>
#include<algorithm>
#include<queue>
#include<stack>//求式子
using namespace std;
typedef long long ll;
ll gcd(ll x,ll y)
{
if(x%y == 0) return y;
else return gcd(y, x%y);
}
int main()
{
int t;
scanf("%d", &t);
while(t--)
{
ll n, up = 0, down, temp = 0;
scanf("%lld", &n);
down = n;
for(int i = 1; i <= n; i++)
{
ll t;
scanf("%lld", &t);
up += t*t;
temp += abs(t);
}
up = up*n - temp*temp;
ll gcd1 = gcd(up, down);
up /= gcd1, down /= gcd1;
printf("%lld/%lld\n", up, down);
}
return 0;
}