区分的関数計算

fabs（x）補足：xの絶対値を返します。

#include<bits/stdc++.h>

using namespace std;

const double PI = 1e-10;

double sin(double x)
{
    
    
    double g = 0;
    double t = x;
    int n = 1;
    do
    {
    
    
        g += t;
        n++;
        t = -t * x * x / (2 * n - 1)/(2 * n - 2);
    }
    while(fabs(t) >= PI);
    return g;
}

int main()
{
    
    
    double k, r, s;
    cin>>r>>s;
    if(r * r <= s * s)
    k = sqrt(sin(r) * sin(r) + sin(s) * sin(s));
    else
    k = sin(r * s) / 2;
    cout<<k<<endl;
    return 0;
}