Three points being collinear means being on the same straight line. The collinearity theorem of points is a common theorem in plane geometry, which states that three different points are collinear if they are on the same straight line. This theorem can also be said to be one of the basic principles of plane geometry and an important basis for studying plane geometry. The three-point collinearity theorem can be proved from the perspective of plane geometry. Suppose there are three different points A, B and C. If these three points are on the same straight line, their positional relationship can be expressed as AC=AB+BC, because straight line AC is equivalent to the sum of the lengths of straight line AB and straight line BC. If this equation holds, it means that point A, point B and point C are on the same straight line. On the other hand, if three points are not collinear, an equation cannot be formed.