What is the measure of interior angle of a regular nonagon?

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A nonagon is a polygon that has nine sides. In the figure below are several types of nonagons.

What is the measure of interior angle of a regular nonagon?

Nonagon classifications

Like other polygons, a nonagon can be classified as regular or irregular. If all the sides and interior angles of a nonagon are equal, it is a regular nonagon. Otherwise it is an irregular nonagon.

Regular nonagonIrregular nonagon
What is the measure of interior angle of a regular nonagon?
What is the measure of interior angle of a regular nonagon?
All sides and interior angles are equal Not all sides and angles are equal

Nonagons and other polygons can also be classified as either convex or concave. If all interior angles of a nonagon are less than 180°, it is convex. If one or more interior angles is larger than 180°, it is concave. A regular nonagon is a convex nonagon.

Convex nonagonConcave nonagon
What is the measure of interior angle of a regular nonagon?
What is the measure of interior angle of a regular nonagon?
All interior angles < 180° One or more interior angles > 180°

Diagonals of nonagon

A diagonal is a line segment joining two non-consecutive vertices. A total of twenty-seven distinct diagonals can be drawn for a nonagon. The following figure is an example.

What is the measure of interior angle of a regular nonagon?

There are 6 diagonals extending from each of the 9 vertices of the nonagon above creating a total of 27 diagonals.

Internal angles of a nonagon

The sum of the interior angles of a nonagon equals 1260°.

What is the measure of interior angle of a regular nonagon?

As shown in the figure above, six diagonals can be drawn to divide the nonagon into seven triangles. The blue lines above show just one way to divide the nonagon into triangles; there are others. The sum of interior angles of the seven triangles equals the sum of interior angles of the nonagon. Since the sum of the interior angles of a triangle is 180°, the sum of the interior angles of the nonagon is 9 × 180° = 1260°.

Regular nonagon

A regular nonagon is a nonagon in which all sides have equal length and all interior angles have equal measure.

Angles of a regular nonagon

Since each of the nine interior angles in a regular nonagon are equal in measure, each interior angle measures 1260° ÷ 9 = 140°, as shown below.

What is the measure of interior angle of a regular nonagon?

Each exterior angle of a regular nonagon has an equal measure of 40°.

What is the measure of interior angle of a regular nonagon?

Symmetry in a regular nonagon

A regular nonagon has 9 lines of symmetry and a rotational symmetry of order 9, meaning that it can be rotated in such a way that it will look the same as the original shape 9 times in 360°.

Lines of symmetryRotational symmetry
What is the measure of interior angle of a regular nonagon?
What is the measure of interior angle of a regular nonagon?
9 lines of symmetry Nine 40° of rotation about the center.


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A three-dimensional space (3D) has three dimensions, such as length, width, and height (or depth). The term "3D" is commonly used to describe shapes and figures in geometry. We live in a 3D world, every object we touch, see, and use are 3D objects. The following is a few examples.

What is the measure of interior angle of a regular nonagon?

3D shapes and figures

While the dimensions of a 2D shape can be described with length and width, a 3D shape requires an additional dimension, often referred to as height or depth.

2D3D
What is the measure of interior angle of a regular nonagon?
What is the measure of interior angle of a regular nonagon?
The dimensions of the 2D rectangle above are length and width.The base of the 3D rectangular prism above is a rectangle that gives it two of its dimensions. The height of the prism is perpendicular to its base giving the prism its third dimension.

3D Coordinate geometry

Determining the position of a point in 3D is similar to determining the position of a point in 2D, except that there is a third axis, the z-axis, in addition to the x- and y-axes. All three axes are perpendicular to each other, as shown in the figure below.

What is the measure of interior angle of a regular nonagon?

While the x- and y-axes in 2D are conventionally the horizontal and vertical axes, respectively, in 3D their orientations can vary. As such, when determining which direction to move along any axis, the negative and positive direction must be indicated on the axes, as in the image above. A negative value indicates movement along an axis in the negative direction, and a positive value indicates movement in the positive direction. As such, the point (2, 3, 4) indicates movement in the positive direction along each axis in the coordinate space above.


See also geometric figures, 1D, 2D.