poj 2601 Simple calculations

/* 惭愧啊，公式推导了老半天 数学不好的伤不起啊。。。 */ #define LOCAL #include<iostream> #include<iomanip> using namespace std; int main() { #ifdef LOCAL freopen("input.txt","r",stdin); freopen("output.txt","w",stdout); #endif int i,n;double x,y,c,sum; while(cin>>n) { sum=0; cin>>x>>y; for(i=1;i<=n;i++) {cin>>c;sum+=2*(n+1-i)*c;} sum=(n*x+y-sum)/(n+1); cout<<setprecision(2)<<setiosflags(ios::fixed)<<sum<<endl; } return 0; }