C Programs

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Decision Making Programs in C

Check whether triangle is equilateral, isosceles, or scalene

Introduction

A triangle can be classified into three types based on its sides:

1. Equilateral Triangle → All three sides are equal.
   Example: a = 5, b = 5, c = 5
2. Isosceles Triangle → Any two sides are equal.
   Example: a = 5, b = 5, c = 3
3. Scalene Triangle → All sides are different.
   Example: a = 4, b = 5, c = 6

Before classification, we must ensure that the three sides form a valid triangle using the triangle inequality theorem:

• a + b > c
• a + c > b
• b + c > a

Only if these conditions are satisfied can we classify the triangle.

C Program: Check Triangle Type

#include <stdio.h>
 
int main() {
    float a, b, c;
 
    // Input sides of triangle
    printf("Enter first side: ");
    scanf("%f", &a);
    printf("Enter second side: ");
    scanf("%f", &b);
    printf("Enter third side: ");
    scanf("%f", &c);
 
    // Check triangle validity
    if ((a + b > c) && (a + c > b) && (b + c > a)) {
 
        // Classify the triangle
        if (a == b && b == c) {
            printf("The triangle is Equilateral.\n");
        }
        else if (a == b || a == c || b == c) {
            printf("The triangle is Isosceles.\n");
        }
        else {
            printf("The triangle is Scalene.\n");
        }
 
    } else {
        printf("The given sides do not form a valid triangle.\n");
    }
 
    return 0;
}
 

 
OUTPUT 1 :
Enter first side: 5
Enter second side: 5
Enter third side: 5
The triangle is Equilateral.

OUTPUT 2 :
Enter first side: 5
Enter second side: 5
Enter third side: 3
The triangle is Isosceles.

OUTPUT 3 :

Enter first side: 3
Enter second side: 4
Enter third side: 5
The triangle is Scalene.

OUTPUT 4 :

Enter first side: 1
Enter second side: 2
Enter third side: 10
The given sides do not form a valid triangle.
 

Explanation

1. The program reads three side lengths: a, b, and c.
2. It checks if the sides satisfy the triangle inequality theorem.
3. If valid:
   • All sides equal → Equilateral.
   • Any two equal → Isosceles.
   • All different → Scalene.
4. If not valid → It displays an error message.