These five kinds are commonly used in high school mathematics. In fact, there are other forms, but high school mathematics is not necessary! Relationship ◆ Slope of straight line: k = (y2-y1)/(x2-x1) (x1≠ x2) (1) General formula: Applicable to all straight lines Ax+By+C=0 (where a) When k does not exist, the straight line can be expressed as x=x0 (3) intercept formula: it is not applicable to the straight line perpendicular to any coordinate axis and the straight line passing through the origin x/a+y/b= 1. Know that the straight line intersects the X axis at (a, 0) and the Y axis at (0). When k < 0, y decreases with the increase of x, and when two straight lines are parallel, k1= k2; When two straight lines are perpendicular, K 1 (5) two-point formula x 1 is not equal to x2 y 1 is not equal to y2 (y-y1) =1