double rk4( double x0, double y0, double h, double (*fun)(double, double))
{
double y1;
double k1,k2,k3,k4;
k1 = h*fun(x0 ,y0 );
k2 = h*fun(x0+h/2.,y0+k1/2.);
k3 = h*fun(x0+h/2.,y0+k2/2.);
k4 = h*fun(x0+h ,y0+k3 );
y1 = y0 + (k1+2.*k2+2.*k3+k4)/6.;
return y1;
}
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