The common expressions of linear equations are a little oblique, oblique, two-point and intercept. When the coordinates of two points on a straight line are known, the two-point expression is often used to express linear equations.
in the two-dimensional coordinate system, the expression formula of two-point formula is (y-y2)/(y1-y2)=(x-x2)/(x1-x2).
The linear equation cannot be expressed by two-point formula, because the denominator of the two-point formula is at this time, and the equation is meaningless. That is, the two-point equation cannot be used to represent the coordinate axis or the straight line parallel to the coordinate axis.