On a coordinate grid, slope tests whether lines are parallel (equal slopes) or perpendicular (slopes that multiply to -1), the distance formula measures any segment, the midpoint averages the endpoints, and a point that divides a segment in a given ratio is found by going that fraction of the way along. With these tools you can prove that four points make a rectangle, that a point lies on a circle, and compute the perimeter and area of a polygon from its vertices. This week the crew tapes a grid on the stage floor and gives every mark coordinates.