新手，求解答此问题，同事望给点学习c++的建议，感觉自己尽力了但是学不好

#include<iostream>
using namespace std;
double tax(int net_income);
int main()
{
    int net_income;
    double tax_bill;
    double five_percent_tax,ten_percent_tax;
    cout<<"enter net income(rounded to whole dollars) $";
    cin>>net_income;
    if(net_income<=15000)
        tax_bill=0;
    else if((net_income>15000)&&(net_income<=25000))
        tax_bill=(0.05*(net_income-15000));
    else
    {
        five_percent_tax=0.05*10000;
        ten_percent_tax=0.10*(net_income-25000);
        tax_bill=(five_percent_tax+ten_percent_tax);
    }
    cout.setf(ios::fixed);
    cout.setf(ios::showpoint);
    cout.precision(2);
    cout<<"net income =$"<<net_income<<endl
        <<"tax bill=$"<<tax_bill<<endl;
    return 0;
}
这个程序里面cout<<"net income =$"<<net_income<<endl
        <<"tax bill=$"<<tax_bill<<endl;中<<"tax bill=$"为什不能写成<<"tax_bill=$"