ax^2 + bx + c = 0 の形の二次方程式を二次方程式を使って解く簡単なCプログラムを紹介しよう：

```
#include <stdio.h>
#include <math.h>

int main() {
    double a, b, c, discriminant, root1, root2;

    printf("Enter coefficients a, b, and c: ");
    scanf("%lf %lf %lf", &a, &b, &c);

    discriminant = b * b - 4 * a * c;

    if (discriminant > 0) {
        root1 = (-b + sqrt(discriminant)) / (2 * a);
        root2 = (-b - sqrt(discriminant)) / (2 * a);
        printf("Two real and distinct roots: %.2lf and %.2lf\n", root1, root2);
    } else if (discriminant == 0) {
        root1 = -b / (2 * a);
        printf("One real and equal root: %.2lf\n", root1);
    } else {
        double realPart = -b / (2 * a);
        double imaginaryPart = sqrt(-discriminant) / (2 * a);
        printf("Two complex roots: %.2lf + %.2lfi and %.2lf - %.2lfi\n", realPart, imaginaryPart, realPart, imaginaryPart);
    }

    return 0;
}
```

このプログラムは係数a, b, cを求め、判別式に基づいて根を計算する。根は判別式の値によって、実数で区別されるもの、実数で等しいもの、複素数として表示される。
Here's a simple C program that can solve a quadratic equation of form ax^2 + bx + c = 0 using the quadratic formula:

```
#include <stdio.h>
#include <math.h>

int main() {
    double a, b, c, discriminant, root1, root2;

    printf("Enter coefficients a, b, and c: ");
    scanf("%lf %lf %lf", &a, &b, &c);

    discriminant = b * b - 4 * a * c;

    if (discriminant > 0) {
        root1 = (-b + sqrt(discriminant)) / (2 * a);
        root2 = (-b - sqrt(discriminant)) / (2 * a);
        printf("Two real and distinct roots: %.2lf and %.2lf\n", root1, root2);
    } else if (discriminant == 0) {
        root1 = -b / (2 * a);
        printf("One real and equal root: %.2lf\n", root1);
    } else {
        double realPart = -b / (2 * a);
        double imaginaryPart = sqrt(-discriminant) / (2 * a);
        printf("Two complex roots: %.2lf + %.2lfi and %.2lf - %.2lfi\n", realPart, imaginaryPart, realPart, imaginaryPart);
    }

    return 0;
}
```

This program will ask for the coefficients a, b, and c, then calculate the roots based on the discriminant. It will display the roots as real and distinct, real and equal, or complex, depending on the value of the discriminant.