The three vertices of a parallelogram are (1, 1), (4, 4) and (4, 8). Find thefourth vertex.

Let A(1, 1), B(4, 4), C(4, 8) and D(x, y) be the vertices of a parallelogram ABCD. Since, the diagonals of a parallelogram bisect each other.
Therefore coordinates of the mid-point of AC = Coordinates of the mid-point of BD.

We know that the midpoint of a line segment AB with A(x, y) and B(u, v) is ⇒ 4 + x = 5, 4 + y = 9
⇒ x = 1, y = 5
Therefore (1, 5) is the fourth vertex.