20. Intro Ex 1

1. Write a function slope-posn that takes in a posn and gives the slope of the line through (0,0) and the posn.

    slope-posn : posn -> number
   

2. Write a function perp? that takes in two posns each being (change in x, change in y) - like above - and returns true if they represent slopes of perpendicular lines.

    perp? : posn posn -> boolean
   

3. Write on-line? that determines if a posn is on a line given in y=mx+b form.

    on-line?: number(m) number(b) posn -> boolean
   

4. Write a function unit that takes in a posn and puts out the posn with each coordinate divided by the distance the input posn is from the origin. For example, the point (3,4) is 5 units from the origin, so the output should be (3/5,4/5).

    unit : posn -> posn
   

You should write your own check-expects, but there are a few to test your code.