In the step shown of the construction of a line through a point that is perpendicular to the given line, why must the compass point be placed on points a and b? how would the construction be different if the compass point were placed at random points on the original line?

Lin is writing an equation to model the proportional relationship between y, the total cost in dollars of downloading videos from a website, and x, the number of videos downloaded. she knows that the total cost to download 3 videos was $12. her work to find the equation is shown below. joylin’s work step 1 k= 3/12= 0.25 step 2 y= 0.25x where did joylin make her first error?