In order for segment AB to be tangent to the circle, angle CAB would have to be 90 degree angle. To determine this, we use the Pythagorean theorem (a^2+b^2=c^2). If it is true, then it is a right angle.
12^2+4^2=13^2
144+14=169
160<169
The Pythagorean theorem in this case does not work, so angle CAB is not a right angle; therefore segment AB is NOT tangent to the circle.
