题目链接：https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=1141

题意：判断区间[a,b]的f（i）是否为素数，f（i）=i*i+i+40；

思路：打个表，然后注意精度；

#include
     
      using namespace std;#define ll long long#define esp 1e-13const int N=1e4+10,M=1e6+50000,inf=1e9+10,mod=1000000007;int prime(int n){    if(n<=1)    return 0;    if(n==2)    return 1;    if(n%2==0)    return 0;    int k, upperBound=n/2;    for(k=3; k<=upperBound; k+=2)    {        upperBound=n/k;        if(n%k==0)            return 0;    }    return 1;}int f(int x){    return x*x+41+x;}int flag[N];int main(){    int x,y,i,z,t;    for(i=0;i<=10000;i++)    if(prime(f(i)))    flag[i+1]=flag[i]+1;    else    flag[i+1]=flag[i];    while(~scanf("%d%d",&x,&y))    {        double sum=flag[y+1]-flag[x];        double di=(y-x+1);        double ans=sum/di*100.0+1e-8;        printf("%.2f\n",ans);    }    return 0;}