How to Understand Angles in Quadrilaterals

Quadrilaterals, the versatile four-sided figures, hold a prominent place in the realm of geometry. While they come in various shapes and sizes, one invariant truth about them is the sum of their interior angles. Join us on an exploration to understand the intricate relationships and properties of angles in these fascinating figures.

How to Understand Angles in Quadrilaterals

Step-by-step Guide: Angles in Quadrilaterals

The Basics of Quadrilaterals:
A quadrilateral, by definition, is a polygon with four sides and four vertices. The sum of its interior angles is always \(360^\circ\).

Types of Quadrilaterals and Their Angles:

  • Square: All angles are right angles, each measuring \(90^\circ\).
  • Rectangle: All angles are right angles, each \(90^\circ\).
  • Rhombus: Opposite angles are equal.
  • Parallelogram: Opposite angles are equal; consecutive angles are supplementary (add to \(180^\circ\)).
  • Trapezoid: Angles adjacent to the parallel sides are supplementary.

Calculating Missing Angles:
Given three angles in a quadrilateral, the fourth can be found by subtracting the sum of the given three angles from \(360^\circ\).

External Angles:
Every angle in a polygon has an external angle. For quadrilaterals, the sum of the external angles is always \(360^\circ\).


Example 1:
Finding a Missing Angle in a Parallelogram
In the parallelogram \( ABCD \), we have the following angles:
Angle \( A = 70^\circ \)
Angle \( B = 110^\circ \)
Angle \( C = 70^\circ \)
Angle \( D =? \)

To find angle \( D \), we use the property that the sum of interior angles in any quadrilateral is \( 360^\circ \).
Angle \( D = 360^\circ – (70^\circ + 110^\circ + 70^\circ) = 360^\circ – 250^\circ = 110^\circ \).
Thus, angle \( D \) in parallelogram \( ABCD \) is \( 110^\circ \).

Example 2:
Angles in a Trapezoid: In the trapezoid \( WXYZ\), where:
Side \( WX\) is parallel to side \( ZY\).
Angle \( W = 80^\circ \)
Angle \( X = 120^\circ \)
Angle \( Z =? \)
Angle \( Y =? \)

For trapezoids, the angles on the same side of the non-parallel line are supplementary. This means they add up to \( 180^\circ \).
Using the supplementary angles property:
Angle \( Y = 180^\circ – 80^\circ = 100^\circ \)
Angle \( Z = 180^\circ – 120^\circ = 60^\circ \).
So, in trapezoid \( WXYZ \), angle \( Y \) is \( 100^\circ \) and angle \( Z \) is \( 60^\circ \).

Practice Questions:

  1. In a rectangle, if one angle measures \(85^\circ\), what are the measures of the other three angles?
  2. In a rhombus, if one angle is \(130^\circ\), what is the measure of its opposite angle?
  3. Given a general quadrilateral with angles \(A\), \(B\), \(C\), and \(D\), if angle \(A = 90^\circ\), angle \(B = 80^\circ\), and angle \(C = 100^\circ\), find angle \(D\).


  1. In a rectangle, all angles are right angles, each measuring \(90^\circ\). Thus, the given angle is not possible for a rectangle.
  2. In a rhombus, opposite angles are equal. Thus, if one angle is \(130^\circ\), its opposite angle is also \(130^\circ\).
  3. Angle \(D = 360^\circ – (90^\circ + 80^\circ + 100^\circ) = 90^\circ\).

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