#include <iostream>
#include <cmath>
 
int fact( int i )
{
	int ret = 1;
	while( i )
	{
		ret *= i--;
	}
	return ret;
}
 
double mycos( double x )
{
	const double eps = 1e-5;
	double res = 0;
	int i = 0;
	double term = 0;
	do
	{
		term = std::pow( x, i*2 ) / fact( 2*i );
		term = i & 1 ? -term : term;
		res += term;
		//std::cout << term << std::endl;
		i++;
	}
	while( std::abs(term) > eps );
	return res;
}
 
int main() {
 
	double x = 0.5;
	std::cout << "mycos( " << x << " ) = " << mycos(x) << std::endl;
	std::cout << "std::cos( " << x << " ) = " << std::cos(x) << std::endl;
 
	return 0;
}

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