c++求方程的根，用3个函数分别求当大于0、等于0和小于0时的根，并输出结果。要求在主函数输入a、b、c的值，输出方程的根。

以下是C++求解一元二次方程的根的程序，其中使用了三个函数分别求解当b^2-4ac大于0、等于0和小于0时的根，并输出结果：

#include <iostream>
#include <cmath>

using namespace std;

void roots_greater_than_zero(double a, double b, double c) {
    double x1 = (-b + sqrt(b * b - 4 * a * c)) / (2 * a);
    double x2 = (-b - sqrt(b * b - 4 * a * c)) / (2 * a);
    cout << "The roots are real and different." << endl;
    cout << "x1 = " << x1 << endl;
    cout << "x2 = " << x2 << endl;
}

void roots_equal_to_zero(double a, double b, double c) {
    double x = -b / (2 * a);
    cout << "The roots are real and same." << endl;
    cout << "x1 = x2 = " << x << endl;
}

void roots_less_than_zero(double a, double b, double c) {
    double realPart = -b / (2 * a);
    double imaginaryPart = sqrt(-b * b + 4 * a * c) / (2 * a);
    cout << "The roots are complex and different." << endl;
    cout << "x1 = " << realPart << "+" << imaginaryPart << "i" << endl;
    cout << "x2 = " << realPart << "-" << imaginaryPart << "i" << endl;
}

int main() {
    double a, b, c;
    cout << "Enter the coefficients a, b and c: ";
    cin >> a >> b >> c;
    double discriminant = b * b - 4 * a * c;
    if (discriminant > 0) {
        roots_greater_than_zero(a, b, c);
    } else if (discriminant == 0) {
        roots_equal_to_zero(a, b, c);
    } else {
        roots_less_than_zero(a, b, c);
    }
    return 0;
}