#include <bits/stdc++.h>
using namespace std;
double xa,xb,ya,yb,x,y;
int main(){
	cin>>xa>>ya>>xb>>yb;
	x=xb-xa;
	y=yb-ya;
	long double len=sqrt(x*x+y*y);
	cout<<fixed<<setprecision(3)<<len<<endl;
	return 0;
}

一共就三个头文件不难背吧，这题目感觉用万能头有点浪费时间

数学学渣没学勾股定理，借鉴了一下前面大佬的代码

#include <iomanip>
#include <cmath>
using namespace std;
double a1,a2,b1,b2,x,y;
int main(){
	cin>>a1>>a2>>b1>>b2;
	x=b1-a1;
	y=b2-a2;
	long double len=sqrt(x*x+y*y);
	cout<<fixed<<setprecision(3)<<len<<endl;
	return 0;
}

#include <bits/stdc++.h>
using namespace std;
int main(int argc, char **argv){
	double x1,y1,x2,y2;
	cin >> x1 >> y1 >> x2 >> y2;
	printf("%.3f",sqrt(pow(x2 - x1,2) + pow(y2 - y1,2)));
	return 0;
}

#include <set>
#include <ios>
#include <list>
#include <cmath>
#include <ctime>
#include <queue>
#include <deque>
#include <stack>
#include <vector>
#include <bitset>
#include <cctype>
#include <cerrno>
#include <cwchar>
#include <cstdio>
#include <fenv.h>
#include <iosfwd>
#include <string>
#include <limits>
#include <math.h>
#include <cstdlib>
#include <iomanip>
#include <clocale>
#include <complex>
#include <cstring>
#include <cstring>
#include <cwctype>
#include <istream>
#include <ostream>
#include <sstream>
#include <fstream>
#include <utility>
#include <stdio.h>
#include <iostream>
#include <stdint.h>
#include <string.h>
#include <tgmath.h>
#include <complex.h>
#include <algorithm>
#include <exception>
#include <stdbool.h>
#include <stdexcept>
#include <streambuf>
#include <functional>
#include <inttypes.h>
#include <bits/stdc++.h>
using namespace std;
int main(){
	double xa,xb,ya,yb,x,y;
	cin>>xa>>ya>>xb>>yb;
	x=abs(xb-xa);
	y=abs(yb-ya);
	cout << fixed << setprecision(3)<<sqrt(x*x+y*y);
	return 0;
}

#include<bits/stdc++.h>
using namespace std;
int main(){
	double Xa,Ya,Xb,Yb;
	cin>>Xa>>Ya>>Xb>>Yb; 
	double AB=sqrt((Xb-Xa)*(Xb-Xa)+(Yb-Ya)*(Yb-Ya)); 
	cout<<fixed<<setprecision(3)<<AB<<"\n";
}

#include<bits/stdc++.h>
using namespace std;
int main(){
	double Xa,Ya,Xb,Yb;//定义 
	cin>>Xa>>Ya>>Xb>>Yb;//输入A,B坐标 
	double AB=sqrt((Xb-Xa)*(Xb-Xa)+(Yb-Ya)*(Yb-Ya));//勾股定理 
	//printf("%.3f",AB); 
	cout<<fixed<<setprecision(3)<<AB<<"\n";
} 

#include <bits/stdc++.h>
using namespace std;
double xa,xb,ya,yb,x,y;
int main(){
	cin>>xa>>ya>>xb>>yb;
	x=xb-xa;
	y=yb-ya;
	long double len=sqrt(x*x+y*y);
	cout<<fixed<<setprecision(3)<<len<<endl;
	return 0;
}

#include <bits/stdc++.h>
using namespace std;
double xa,xb,ya,yb,m,n;
int main(){
	cin>>xa>>ya>>xb>>yb;
	m=xb-xa;
	n=yb-ya;
	 double an=sqrt(m*m+n*n);
	cout<<fixed<<setprecision(3)<<an<<endl;
	return 0;
}家人们谁懂不知道用哪个直接用玩呢个头

#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
int main() {
    double xa,ya,xb,yb,a,b,c,d;
    cin>>xa>>ya>>xb>>yb;//四个坐标
    a=xb-xa;//直角边1
    b=yb-ya;//直角边2
    c=a*a+b*b;
    cout << fixed << setprecision(3)<<sqrt(a*a+b*b);//勾股定理
    return 0;
}