arctanx

arctanx=x-x^3/3+x^5/5-x^7/7+.......

#include<iostream>
using namespace std;
double arctan(double x){
    double sqr=x*x;
    double e=x;
    double r=0;
    int i=1;
    while(e/i>1e-15){
        double f=e/i;
        r=(i%4==1)? r+f : r-f ;
        e=e*sqr;
        i+=2;
    }
    return r;
}
int main(){
    double a=arctan(1/5.0);//注意要寫成5.0，否則取整為0
    cout<<a<<endl;

    return 0;
}
 

sinx

sinx=x/1!-x^3/3!+x^5/5!-x^7/7!;

#include<iostream>
#include<cmath>

using namespace std;
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)>=1e-6);
    return g;
}
int main(){
    double a;
    cin>>a;

    cout<<sin(a)<<endl;

    return 0;
}