Return true if three line are concurrent , else false .
bool checkConcurrent ( int a1 , int b1 , int c1 , int a2 , int b2 , int c2 , int a3 , int b3 , int c3 ) { return ( a3 * ( b1 * c2 - b2 * c1 ) + b3 * ( c1 * a2 - c2 * a1 ) + c3 * ( a1 * b2 - a2 * b1 ) == 0 ) ; }