#include <stdio.h>
#include <math.h>
 
// 三角形の構造体を定義
typedef struct {
    double a; // 1つ目の辺の長さ
    double b; // 2つ目の辺の長さ
    double c; // 3つ目の辺の長さ
    double area; // 三角形の面積
} list;
 
int main(void) {
    // 構造体変数を宣言
    list date;
 
    // 辺の長さを設定
    date.a = 5.0;
    date.b = 5.0;
    date.c = 5.0;
 
    // 半周の計算
    double s = (date.a + date.b + date.c) / 2.0;
 
    // ヘロンの公式で面積を計算
    date.area = sqrt(s * (s - date.a) * (s - date.b) * (s - date.c));
 
    // 結果の出力
    printf("a : %lf\n", date.a);
    printf("b : %lf\n", date.b);
    printf("c : %lf\n", date.c);
    printf("三角形の面積 : %lf\n", date.area);
 
    return 0;
}

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