关于c++求根程序

用弦截发求方程的根：f(x)=x^3-5*x^2+16*x-80=0,就是通过判断f(x1)*f(x2)的符号逼近根的方法，下面是程序error C2064: term does not evaluate to a function请指教：#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
double f(double);
double xpoint(double,double);
double root(double,double);
double f1,f2;
int main()
{
double x,x1,x2;
do
{cout<<"input x1,x2:"<<endl;
cin>>x1>>x2;
f1=f(x1);
f2=f(x2);}
while (f1*f2>=0);
x=root(x1,x2);
cout<<setiosflags(ios::fixed)<<setprecision(7);
cout<<"a root of the equation is"<<x<<endl;
return 0;
}

double f(double x)
{
double y;
y=x*x*x-5*x*x+16*x-80;
return y;
}

double xpoint(double x1,double x2)
{
double x;
x=(x1*f2-x2*f1)/(f2-f1);
return x;
}