equation: y-y1=k(x-x1)

among them (x1, y1) is the coordinate of a point passing the straight line on the coordinate system, and k is the slope of the straight line.

derivation: if the straight line l1 passes through the point p1 (x1, y1), and the slope is k, find the equation of l1.

suppose point p(x, y) is any point on the straight line different from point p1. the slope of straight line pp1 should be equal to the slope of straight line l1. according to the slope formula of a straight line passing through two points, k=(y-y1)/( x－x1) (and: x≠x1)

therefore, straight line l1: y-y1=k(x-x1)