findroot

double findroot(double lb, double ub, double eps2, double(*pfunc)(double)){
    double mid, f;
    while(ub > lb){
        mid = (lb + ub) / 2;
        f = pfunc(mid);
        if(fabs(f) < eps2){
            return mid;
        }
        else if(f >= eps2){
            lb = mid;
        }
        else{
            ub = mid;
        }
    }
    return lb;
}