Understanding the Four Angles of a Quadrilateral

Understanding angles of quadrilateral is essential for solving key questions in geometry, both for school board exams and competitive tests. Knowing angle properties makes it easier to tackle real-world construction problems, prove theorems, and analyse different shapes confidently. It forms the basis of many advanced maths topics too.

Formula Used in Angles of Quadrilateral

The standard formula is: \( \text{Sum of interior angles} = (n - 2) \times 180^\circ \), where \( n \) is the number of sides. For a quadrilateral (\( n = 4 \)), sum = \( (4-2) \times 180^\circ = 360^\circ \).

Here’s a helpful table to understand angles of quadrilateral more clearly:

Angles of Quadrilateral Table

Angle Position	Value (for Rectangle/Square)	Property Applies?
Angle A	90°	Yes
Angle B	90°	Yes
Angle C	90°	Yes
Angle D	90°	Yes
Sum of All Angles	360°	Always

This table shows how the pattern of angles of quadrilateral appears regularly, especially for common shapes like rectangles and squares.

Properties and Types of Angles in a Quadrilateral

A quadrilateral always has four interior angles, labelled as A, B, C, and D. No matter the shape—parallelogram, rhombus, trapezium, rectangle or kite—the interior angles always add up to 360°. Each angle can be different, but their sum remains the same. The exterior angle at any vertex is calculated as \( 180^\circ - \text{Interior Angle} \). For rectangles and squares, each angle is 90°. Parallelograms, like those explained in Angles of Parallelogram, have opposite angles equal.

Special rule: In a cyclic quadrilateral (where the quadrilateral is inscribed inside a circle), the sum of the two opposite angles is always 180°. This is a key theorem used in many geometry problems.

How to Find an Unknown Angle in a Quadrilateral

Follow these steps to calculate an unknown angle if three are known:

1. Write the angle sum formula: \( A + B + C + D = 360^\circ \).

2. Add together the three known angles.

3. Subtract their total from 360° to find the fourth angle.

4. For exterior angles, subtract the interior angle from 180° as per the exterior angle rule. (Learn more about exterior angles at Exterior Angle Theorem.)

Worked Example – Solving a Problem

Let’s solve for the missing angle in a quadrilateral where three interior angles are 75°, 120°, and 90°:

1. Add the known angles: \( 75^\circ + 120^\circ + 90^\circ = 285^\circ \).

2. Subtract this sum from the total (360°): \( 360^\circ - 285^\circ = 75^\circ \).

3. The fourth angle is 75°.

Suppose you want to find the exterior angle at the vertex where the interior angle is 90°:
Exterior angle = \( 180^\circ - 90^\circ = 90^\circ \).

Practice Problems

• Find the fourth angle of a quadrilateral if three angles are 80°, 95°, and 115°.
• If one angle of a quadrilateral is 100°, what is the measure of its exterior angle?
• In a cyclic quadrilateral, one angle is 110°. What is the measure of its opposite angle?
• A parallelogram has one angle measuring 70°. What are the measures of the other three angles?

Common Mistakes to Avoid

• Forgetting that the sum of all interior angles in a quadrilateral is always 360°, not 180° (the latter is for triangles; more at Angle Sum Triangle).
• Getting confused between interior and exterior angle definitions.
• Not checking if the given quadrilateral is special (like cyclic), which can add more properties for opposite angles.
• Mixing up sum rules for polygons with more than four sides (see the comparison in Angles in a Pentagon).

Real-World Applications

Angles of quadrilateral are used in engineering, architecture, and design to ensure that shapes fit together perfectly. For example, finding the proper angles allows builders to create accurate floors, bridges, and walls. Learning these rules with Vedantu prepares students for practical geometry and competitive maths.

We explored the idea of angles of quadrilateral, key formulas, stepwise solutions, and real-life uses. Keep practising with Vedantu, and try more questions to strengthen your grasp of these important geometry skills. For more on properties and types of quadrilaterals, visit Quadrilateral or deepen your knowledge about angle sums at Angle Sum Property of Quadrilateral.