二分法求解表达式的值

用C/C++实现，通过，二分法求解表达式的值：

#include <bits/stdc++.h>

using namespace std;
int ZP_MAX = 1000;
string str;

double calc(double x)
{
    return (5*x+6)*9-(2*x+3);
}

int main()
{

    // 二分法, x1 = 0, x2 = 20
    double x1 = -30.0, x2 = 30.0;

    while(1)
    {
        double temp = (x1 + x2) / 2.0;
        double ans = calc(temp) - 0;

        if (fabs(ans) < 1e-5)
        {
            printf("%.2lf
", temp);
            break;
        }
        if (ans > 0)
            x2 = temp;
        if (ans < 0)
            x1 = temp;
    }
    return 0;
}