15.(C) The area of an isosceles triangle(isosceles is refer to the triangle which two shape are equal)

 #include<stdio.h>

#include<math.h>

int main(){  

   int a,b;

   float area;  // I will has to learn the use of sqrt

   // ½[√(a2 − b2 ⁄4) × b]

   scanf("%d",&a);

   scanf("%d",&b);

   area=0.5*b*pow((pow(a,2)-pow(b,2)/4),0.5);

   printf("The area is %f", area);

   return 0;

}