Generally, in A Plane cartesian coordinate system, the straight line L passes through points A(X1, y1) and B(x2,y2), where x1≠x2, then AB=(x2-x1,y2-y1) is the vector of L one direction, straight line L slope k=(y2-y1)/(x2-x1), and then k=tanα (0≤α<π), the straight line L slope Angle α can be obtained. Let's call tanα=k, Equation y-y0=k(x-x0) straight line point-slope form Equation, where (x0,y0) is a Point on the straight line.

When α is π/2, i.e. (90 degrees, straight line and X-axis perpendicular), tanα is meaningless and there is no Point-slope form Equation.

Point-slope form Equation is commonly used in derivation number, The tangent Equation is currently used by taking a point on the tangent line and a number of Equation derivatives of the curve (a tangent slope of a point on the Equation). It is suitable for solving the straight line Equation problem by knowing the coordinates of one point and the straight line slope.