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Based on the measure of their interior angles, all quadrilaterals are classified into two groups: convex and concave.

## What is a Convex Quadrilateral

A convex quadrilateral is a quadrilateral having all its interior angles measuring less than 180°. Both the diagonals of a convex quadrilateral lie inside the closed figure.

Examples: A square, a rectangle, a parallelogram, a rhombus, a trapezoid, and a kite.

In the given convex quadrilateral ABCD, all its four interior angles ∠ABC, ∠BCD, ∠CDA, and ∠DAB measure less than 180°. Also the diagonals AC and DB are found to reside inside the closed figure.

Given is a quadrilateral ABCD, having ∠ABC = 80°, ∠BCD = 70°, ∠CDA = 110°. Find the measure of the ∠DAB and state whether it is a convex quadrilateral.

Solution:

As we know,
Sum of the interior angles in a convex quadrilateral = 360°
Thus, in quadrilateral ABCD, ∠ABC + ∠BCD + ∠CDA + ∠DAB = 360°
=> 80° + 70° + 110° + ∠DAB = 360°
=> 280° + ∠DAB = 360°
=> ∠DAB = 360° – 260°
=> ∠DAB = 100°

Since all the interior angles of the given quadrilateral ABCD measure less than 180°, it is a convex quadrilateral.

## What is a Concave Quadrilateral

A concave quadrilateral is a quadrilateral having at least one of its interior angles measuring more than 180°. One of the diagonals of a concave quadrilateral lies outside the closed figure. Any quadrilateral-shape that is not convex is thus a concave quadrilateral.

Example: A dart or an arrowhead.

In the given concave quadrilateral ABCD, ∠BCD measures more than 180°, and the diagonal BD lies outside the closed figure.

Given is a quadrilateral ABCD, having ∠ABC = 30°, ∠DAB = 80°, and ∠CDA = 25°. Find the measure of the ∠BCD and state whether it is a concave quadrilateral.

Solution:

As we know,
Sum of the interior angles in a concave quadrilateral = 360°
Thus, in quadrilateral ABCD, ∠ABC + ∠BCD + ∠CDA + ∠DAB = 360°
=> 30° + ∠BCD + 25° + 80° = 360°
=> ∠BCD + 135° = 360°
=> ∠BCD = 360° – 135°
=> ∠BCD = 225°

Since one of the interior angles of the given quadrilateral ABCD measure more than 180°, it is a concave quadrilateral.

## FAQs

Q1. What is the difference between a convex and a concave quadrilateral?

Ans. A convex quadrilateral has all its interior angles measuring less than 180° and has both the diagonals lying inside the closed figure. In contrast, a concave quadrilateral has one of its interior angle measuring more than 180° and one of its diagonals lie outside the closed figure.

Q2. Why rectangle is a convex quadrilateral?

Ans. Since all the four interior angles of a rectangle measure less than 180°and both diagonals lie wholly inside the closed shape, it is a convex quadrilateral.

Q3. Can a concave quadrilateral be regular?

Ans. Since all the interior angles in a concave quadrilateral are not equal (one of its interior angles measuring more than 180°) it can never be a regular quadrilateral where all the interior angles are of equal measure.