( x2 - x1 ) ^ 2 + ( y2 - y1 ) ^ 2 <= ( r1 + r2 ) ^ 2 for 2 circles to intersect
return sqrt ( ( X [ 0 ] - X [ 1 ] ) * ( X [ 0 ] - X [ 1 ] ) + ( Y [ 0 ] - Y [ 1 ] ) * ( Y [ 0 ] - Y [ 1 ] ) ) <= t * ( V [ 0 ] + V [ 1 ] ) ; } }