#include <stdio.h>
#include <stdlib.h>

/* A utility function to calculate area of triangle formed by (x1, y1), (x2, y2) and (x3, y3) */
float area(int x1, int y1, int x2, int y2, int x3, int y3) 
{ 
    return abs((x1*(y2-y3) + x2*(y3-y1)+ x3*(y1-y2))/2.0); 
} 

​

/* A function to check whether point P(x, y) lies inside the triangle formed by A(x1, y1), B(x2, y2) and C(x3, y3) */
_Bool isInside(int x1, int y1, int x2, int y2, int x3, int y3, int x, int y) 
{ 
    /* Calculate area of triangle ABC */
    float A = area (x1, y1, x2, y2, x3, y3); 

​

    /* Calculate area of triangle PBC */
    float A1 = area (x, y, x2, y2, x3, y3); 

​

    /* Calculate area of triangle PAC */
    float A2 = area (x1, y1, x, y, x3, y3);

 

    /* Calculate area of triangle PAB */    
    float A3 = area (x1, y1, x2, y2, x, y); 
    
    /* Check if sum of A1, A2 and A3 is same as A */
    return (A == A1 + A2 + A3); 
} 

​

/* Driver program to test above function */
int main() 
{ 
    /* Let us check whether the point P(10, 15) lies inside the triangle formed by A(0, 0), B(20, 0) and C(10, 30) */
    if (isInside(0, 0, 20, 0, 10, 30, 10, 15)) 
        printf ("Inside"); 
    else
        printf ("Not Inside"); 

    return 0; 
}