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Properties and Proofs with Rectangles
Example 1 Diagonals of a Rectangle
ALGEBRA Quadrilateral ABCD is a rectangle. If AC = 4 x - 13 and BD = 2 x + 14, find x and
the length of BD.
The diagonals of a rectangle are congruent,
so AC BD.
AC BD Diagonals of a rectangle are. AC = BD Definition of congruent segments. 4 x - 13 = 2 x + 14 Substitution 2 x - 13 = 14 Subtract 2 x from each side. 2 x = 27 Add 13 to each side. x =
27 2 Divide each side by 2.
Example 2 Angles of a Rectangle
ALGEBRA Quadrilateral PQRS is a rectangle. a. Find x****.
QPS is a right angle, so m QPS = 90.
m QPR + m RPS = m QPS Angle Addition Postulate 70 - 4 x + 18 x - 8 = 90 Substitution 62 + 14 x = 90 Simplify. 14 x = 28 Subtract 62 from each side. x = 2 Divide each side by 14.
b. Find y****. Since a rectangle is a parallelogram, opposite sides are parallel. So, alternate interior angles are congruent.
PQS QSR Alternate Interior Angles Theorem m PQS = m QSR Definition of congruent angles 7 y + 6 = y^2 - 2 Substitution 0 = y^2 - 7 y - 8 Subtract 7 y and 6 from each side. 0 = ( y -8)( y + 1) Factor.
y - 8 = 0 y + 1 = 0 y = 8 y = -1 Disregard y = -1 because it yields angle measures less than 0.
BD = 2 x + 14
= 2
= 41 Simplify.
Example 3 Diagonals of a Parallelogram
CONSTRUCTION The Owens family is building a deck in their back yard. Mrs. Owens has laid out stakes where the corners of the deck will be. She has made sure that the opposite sides are congruent. If she measures the diagonals and they are congruent, how can Mrs. Owens be sure that the deck will be a rectangle? Explain. Draw a diagram. We know that
NM OP , NO MP , and NP MO.
Because NM OP and NO MP , MNOP is a parallelogram.
NP and MO are diagonals and they are congruent. A parallelogram with congruent diagonals is a rectangle. So, Mrs. Owens can be sure that the deck is a rectangle.
Example 4 Rectangle on a Coordinate Plane
COORDINATE GEOMETRY Quadrilateral ABCD has vertices A (-6, 9), B (4, 7), C (3, 2), and D (-7, 4). Determine whether ABCD is a rectangle.
Method 1: Use the Slope Formula, m =
y 2 โ y 1 x 2 โ x 1 , to see if
opposite sides are parallel and consecutive sides are perpendicular.
slope of AB = 7 - 9 4 - (-6) or -
1 5
slope of DC = 2 - 4 3 - (-7) or -
1 5
slope of AD = 4 - 9 -7 - (-6) or 5
slope of BC = 2 - 7 3 - 4 or 5
AB โ DC and AD โ BC. Quadrilateral ABCD is a parallelogram.
The product of the slopes of consecutive sides is -1. This means that AB AD , AB BC , BC DC ,
and AD DC. The perpendicular segments create four right angles. Therefore, by definition ABCD is a
rectangle.
