#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

#include<stdio.h>

int main(){

    float b1,b2,h;
    float area;

    printf("Enter the size of two bases and height of the trapezium : ");
    scanf("%f%f%f",&b1,&b2,&h);

    area = 0.5 * ( b1 + b2 ) * h ;
    printf("Area of trapezium is: %.3f",area);
    return 0;
}
Sample output:
Enter the size of two bases and height of the trapezium: 5 8 3
Area of trapezium is: 19.500

#include<stdio.h>

int main(){

    float a;
    float surface_area,volume;

    printf("Enter size of any side of a cube : ");
    scanf("%f",&a);

    surface_area = 6 * (a * a);
    volume = a * a * a;

    printf("Surface area of cube is: %.3f",surface_area);
    printf("\nVolume of cube is : %.3f",volume);

    return 0;
}
Sample output:
Enter size of any side of a cube: 3
Surface area of cube is: 54.000
Volume of cube is: 27.000