#include <stdio.h>
#define PI 3.141
int main(){
  float r, a;
  printf("Radius: ");
  scanf("%f", &r);
  a = PI * r * r;
  printf("%f\n", a);
  return 0;
}

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

int main(){
   
    float a,b,c;
    float s,area;
   
    printf("Enter size of each sides of triangle");
    scanf("%f%f%f",&a,&b,&c);
   
    s = (a+b+c)/2;
    area = sqrt(s*(s-a)*(s-b)*(s-c));
   
    printf("Area of triangle is: %.3f",area);
   
    return 0;
}
Sample output:
Enter size of each sides of the triangle: 2 4 5
Area of triangle is: 3.800

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

int main(){

    float a;
    float area;

    printf("Enter size of side of the equilateral triangle : ");
    scanf("%f",&a);

    area = sqrt(3)/4*(a*a);

    printf("Area of equilateral triangle is: %.3f",area);

    return 0;
}
Sample output:
Enter size of side of the equilateral triangle: 5
Area of equilateral triangle is: 10.825

#include<stdio.h>
int main(){
    float h,w;
    float area;

    printf("Enter height and width of the right angled triangle : ");
    scanf("%f%f",&h,&w);

    area = 0.5 * h * w;
    printf("Area of right angled triangle is: %.3f",area);
    return 0;
}
Sample output:
Enter height and width of the right angled triangle: 10 5
Area of right angled triangle is: 25.000

#include<stdio.h>

int main(){

    float l,w;
    float area;

    printf("Enter size of each sides of the rectangle : ");
    scanf("%f%f",&l,&w);

    area = l * w;
    printf("Area of rectangle is: %.3f",area);

    return 0;
}
[b]Sample output:[b]
Enter size of each sides of the rectangle: 5.2 20
Area of rectangle is: 104.000