C Program to Find Area of a Circle Triangle and Rectangle

It’s time to develop some small programs using operators, printf() and scanf() function. We will write a C program to find the area of circle, area of a Circle using Symbolic Constant, area of a Rectangle, area of a right-angle triangle, and area of a triangle having three sides.

Calculate Area of Circle using C

If the radius of a circle is r then Area is given by ℼ*r²

#include<stdio.h>
int main()
{
     // declare variables
     float radius, area;

     // take inputs
     printf("Enter Radius of Circle(in cm): ");
     scanf("%f", &radius);

     // calculate area
     area = 3.14 * radius * radius;

     // display result
     printf("Area of Circle = %.2f cm\n",
                             radius,area);

     return 0;
}

Output:-

Enter Radius of Circle(in cm): 9.5
Area of Circle = 283.39 cm

Find Area of a Circle Using Symbolic Constant

Here, We will use the symbolic constant to store the value of ℼ, outside of the main() function.

#include<stdio.h>
#define PI 3.14
int main()
{

     // declare variables
     float radius, area;

     // take input
     printf("Enter Radius of Circle(in cm): ");
     scanf("%f",&radius);

     // calculate area
     area = PI * radius * radius;

     // display result
     printf("Area of Circle = %.2f cm\n",
                          radius,area);

     return 0;
 }

Notice that there is no semicolon at the end of a #define line. Value of ℼ taken from the defined line (PI=3.14). We used %.2f, which will print as a floating point, 2 characters after decimal points.

C program to find the area of a Rectangle

Area of Rectangle is given by length * width. Here we take len and wid variable to store the value of length and width respectively.

#include<stdio.h>
int main()
{

     // declare variables
     float len, wid, area;

     // take inputs
     printf("Enter length & width of Rectangle (in cm): ");
     scanf("%f %f",&len,&wid);

     // calculate area
     area = len * wid;

     // display result
     printf("Area of Rectangle= %.3f cm\n",area);

     return 0;
}

Output:-

Enter length & width of Rectangle (in cm): 12.5 15.2
Area of Rectangle= 190.000 cm

C Program to find Area of Right angle triangle

Area of Right angle triangle or Triangle whose base and height is known, their area is given by, Area=(1/2) * base * height Or, 0.5 * base * height

#include<stdio.h>
int main()
{
     // declare variables
     float b, h, area;

     // take inputs
     printf("Enter base and height (in cm): ");
     scanf("%f %f", &b, &h);

     // calculate area
     area = (0.5 * b * h);

     // display result
     printf("Area of triangle= %.2f cm\n",
                                   area );

     return 0;
}

Output:-

Enter base and height (in cm): 22 5.9
Area of triangle= 64.90 cm

Here we used b and h variables for base and height respectively.

Area of a triangle having three sides

Triangle having three sides their area is given by Heron’s Formula for the area of a triangle.

If a, b and c are sides of triangles then from Heron’s Formula,

#include<stdio.h>
#include<math.h>
int main()
{

     // declare variables
     float a, b, c, s, area;

     // take inputs
     printf("Enter a,b and c value: ");
     scanf("%f %f %f",&a,&b,&c);

     // calculate are
     s = (a+b+c)/2;
     area = sqrt( s*(s-a)*(s-b)*(s-c) );

     // display result
     printf("Area = %.2f\n",area);

     return 0;
}

Output:-

Enter a,b and c value: 4.5 8.9 12
Area = 16.64

a, b and c are three sides of the triangle. Here we used sqrt() function to find square root value. sqrt() defined under math.h

If you are using Linux/Unix OS and getting an error then read it Getting “undefined reference to sqrt (or other mathematical functions)” even include math.h header?

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