用c++代码写求方程 的根，用三个函数分别求当b^2-4ac大于0、等于0、和小于0时的根，并输出结果。从主函数输入a、b、c的值。

以下是C++代码实现：

#include <iostream>
#include <cmath>

using namespace std;

void roots_positive(double a, double b, double c) {
    double root1 = (-b + sqrt(b*b - 4*a*c)) / (2*a);
    double root2 = (-b - sqrt(b*b - 4*a*c)) / (2*a);
    cout << "The roots are real and different." << endl;
    cout << "Root 1: " << root1 << endl;
    cout << "Root 2: " << root2 << endl;
}

void roots_zero(double a, double b, double c) {
    double root = -b / (2*a);
    cout << "The roots are real and equal." << endl;
    cout << "Root: " << root << endl;
}

void roots_negative(double a, double b, double c) {
    double realPart = -b / (2*a);
    double imagPart = sqrt(-b*b + 4*a*c) / (2*a);
    cout << "The roots are complex and different." << endl;
    cout << "Root 1: " << realPart << " + " << imagPart << "i" << endl;
    cout << "Root 2: " << realPart << " - " << imagPart << "i" << endl;
}

int main() {
    double a, b, c;
    cout << "Enter the coefficients of the quadratic equation: " << endl;
    cin >> a >> b >> c;
    double d = b*b - 4*a*c;
    if (d > 0) {
        roots_positive(a, b, c);
    } else if (d == 0) {
        roots_zero(a, b, c);
    } else {
        roots_negative(a, b, c);
    }
    return 0;
}

在这个程序中，我们定义了三个函数，分别计算当 $b^2-4ac$ 大于0、等于0、和小于0时的根，并输出结果。主函数中，我们从用户输入获取 $a$、$b$、$c$ 的值，计算 $b^2-4ac$，并根据 $b^2-4ac$ 的值调用不同的函数来计算和输出根。