Diferenciacion e integracion beta 0.1, C++ (gcc)

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//g++  5.4.0
#include<math.h>
#include <iostream>
#include <iomanip>
#include<stdlib.h>
#include<stdio.h>
#include<vector>

using namespace std;

int programa3();
int diferencias(vector<double>,vector<double>,int);
int llenar_datos(vector<double>,vector<double>,int);
    
int main()
{
    cout<<fixed<<setprecision(6);
    programa3();
}

int programa3(){
    int t;
    vector<double> x(1);
    vector<double> fx(1);
    double h;
        
    cin>>t;
    
    llenar_datos(x,fx,t);
    
    diferencias(x,fx,t);
    
}

int llenar_datos(vector<double> x,vector<double> fx ,int t){
    
    double d1, d2;
    cout<<"Hola";
    
    x.resize(t);
    fx.resize(t);
    
    x.clear();
    fx.clear();
    
    for(int i=0; i<t; i++){
        cin>>d1;
        cin>>d2;
        x[i]=d1;
        fx[i]=d2;
    }
}

int diferencias(vector<double> x,vector<double> fx ,int t){
    
    vector<double> dif,dif2;
    double d1, d2, h, a, b;
    
    cin>>a;
    cin>>b;
    
    dif.clear();
    dif2.clear();
        
    dif.resize(t);
    dif2.resize(t);
    
    h=x[1]-x[0];    
    
    for(int i=1; i<t-1; i++){
        dif[i]=(fx[i+1]-fx[i-1])/(2*h);
        dif2[i]=(fx[i+1]-2*fx[i]+fx[i-1])/(h*h);
    }
    cout<<"-----------------------------------------\n";
    cout<<"|x        |"<<"f(x)     |"<<"f'(x)     |"<<"f''(x)  |"<<"\n";
    cout<<"-----------------------------------------\n";
    for(int i=0; i<t; i++){
        cout<<"|"<<x[i]<<" |"<<fx[i]<<" |";
        if(x[i] >= a and x[i] <= b) cout<<dif[i]<<" |"<<dif2[i]<<"|\n";
        else cout<<"          |        |\n";
    }
    cout<<"-----------------------------------------\n";
    
}

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Compilation time: 0.53 sec, absolute running time: 0.17 sec, cpu time: 0.11 sec, memory peak: 3 Mb, absolute service time: 0,71 sec