In the xy-plane shown, square ABCD has its diagonals on the x- and y-axes. What is the area, in square units, of the square?
20
25
50
100
Choice C is correct. The two diagonals of square ABCD divide the square into 4 congruent right triangles, where each triangle has a vertex at the origin of the graph shown. The formula for the area of a triangle is , where b is the base length of the triangle and h is the height of the triangle. Each of the 4 congruent right triangles has a height of 5 units and a base length of 5 units. Therefore, the area of each triangle is
, or 12.5 square units. Since the 4 right triangles are congruent, the area of each is
of the area of square ABCD. It follows that the area of the square ABCD is equal to
, or 50 square units.
Choices A and D are incorrect and may result from using 5 or 25, respectively, as the area of one of the 4 congruent right triangles formed by diagonals of square ABCD. However, the area of these triangles is 12.5. Choice B is incorrect and may result from using 5 as the length of one side of square ABCD. However, the length of a side of square ABCD is .