这道题用到小数点后几位，我忘了头文件。头文件是#include[HTML_REMOVED]，对应的语句是cout << fixed << setprecision(2) << x+y;还可以用printf，printf(“%.2lf”,x+y);
这道题有两种写法：
方法1：

#include<iostream>
#include<iomanip>
using namespace std;
double add(double x, double y)
{
    cout << fixed << setprecision(2) << x+y;
}
int main()
{
    //freopen("xxx.in","r",stdin);
    //freopen("yyy.out","w",stdout);
    double x,y;
    cin >> x >> y;
    add(x,y);
    //fclose(stdin);
    //fclose(stdout);
    return 0;
}

方法2：

#include<iostream>
#include<iomanip>
using namespace std;
double add(double x, double y)
{
    printf("%.2lf",x+y);//cout << fixed << setprecision(2) << x+y;
}
int main()
{
    //freopen("xxx.in","r",stdin);
    //freopen("yyy.out","w",stdout);
    double x,y;
    cin >> x >> y;
    add(x,y);
    //fclose(stdin);
    //fclose(stdout);
    return 0;
}