Question

If (3,17) and (9,-4) are the coordinates of two opposite vertices of a rectangle, what is the sum of the y-coordinates of the other two vertices?

Answer 1

Diagonals of a rectangle bisect each other, so the midpoint of (3,17) and (9,-4) is also the midpoint of the other diagonal. The y-coordinate of the midpoint is (17 + (−4))/2 = 13/2, so the sum of the y-coordinates of the other two vertices is 2·(13/2) = 13.