Big Ideas Math Geometry, 2014
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Big Ideas Math Geometry, 2014 View details
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Exercise 13 Page 411

Write the definition of a square. Then, recall all the theorems you've learned in the chapter and pick the ones that are relevant.

See solution.

Practice makes perfect

By definition, a square is a parallelogram with four congruent sides and four right angles. That is, it is both a rhombus and a rectangle simultaneously.

Therefore, to prove that a quadrilateral is a square, first we need to prove that it is a parallelogram, then prove that it is a rhombus, and finally prove that it is a rectangle.

Proving That It Is a Parallelogram

To prove that a quadrilateral is a parallelogram, we must verify one of the following conditions.
  1. Both pairs of opposite sides of are congruent.
  2. Both pairs of opposite angles are congruent.
  3. One pair of opposite sides are congruent and parallel.
  4. The diagonals bisect each other.

If any of the questions in the diagram above has Yes as the answer, then the quadrilateral ABCD is a parallelogram.

Proving That It Is a Rhombus

To prove that a parallelogram is a rhombus, we must check one of the two conditions below.

  1. It has four congruent sides.
  2. Its diagonals are perpendicular.
  3. Each diagonal bisects a pair of opposite angles.
As before, if the answer to any of the two questions is Yes, then the parallelogram is a rhombus.

Proving That It Is a Rectangle

Finally, to prove that a parallelogram is a rectangle, we must verify either that it has four right angles or that its diagonals are congruent. Let's review the Rectangle Diagonals Theorem.

Rectangle Diagonals Theorem

A parallelogram is a rectangle if and only if its diagonals are congruent.