Answer

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P = 4
A = 1.2310734148701
Sides (Edges) = 10
Faces = 1
Vertices = 10

↓Steps Explained:↓

The Perimeter of an decagon is 4

Calculate side, area

Calculate side:

s =	P
	10

s =	4
	10

s = 0.4

Calculate Area:

A = 5/2s²√5 + 2√5

A = 2.5(0.4)²(3.0776835371753)

A = 2.5(0.16)(3.0776835371753)

A = 1.2310734148701

Calculate Sum of the Interior Angles:

Interior Angle Sum = (sides - 2) x 180°

Interior Angle Sum = (10 - 2) x 180°

Interior Angle Sum = 8 x 180°

Interior Angle sum = 1440°

Calculate decagon diagonals

Diagonals =	n(n - 3)
	2

Diagonals =	10(10 - 3)
	2

Diagonals =	10(7)
	2

Diagonals =	70
	2

Diagonals = 35

Calculate diagonals from 1 vertex:

1 vertex Diagonals = n - 3

1 vertex Diagonals = 10 - 3

1 vertex Diagonals = 7

Calculate number of Δ from 1 vertex:

Triangles = N - 2

Triangles = 10 - 2

Triangles = 8

Other Properties:

Sides (Edges) = 10

Faces = 1

Vertices = 10