# Octagonal pyramid

Find the volume of a regular octagonal pyramid with height v = 100 and the angle of the side edge with the plane of the base is α = 60°.

Result

V =  314269.681

#### Solution:

$v = 100 \ \\ α = 60 \ ^\circ \ \\ α_{1} = α = 60^\circ \doteq 1.0472 \ \\ n = 8 \ \\ \ \\ \tan α = v/r \ \\ \ \\ r = v / \tan (α_{1}) = 100 / \tan 1.0472 \doteq 57.735 \ \\ \ \\ β = \dfrac{ 2 \pi }{ 2 \cdot \ n } = \dfrac{ 2 \cdot \ 3.1416 }{ 2 \cdot \ 8 } \doteq 0.3927 \ rad \ \\ \ \\ \sin β = x / r \ \\ \ \\ x = r \cdot \ \sin (β) = 57.735 \cdot \ \sin 0.3927 \doteq 22.0942 \ \\ \ \\ r^2 = x^2 + w^2 \ \\ w = \sqrt{ r^2-x^2 } = \sqrt{ 57.735^2-22.0942^2 } \doteq 53.3402 \ \\ \ \\ S_{ 1 } = \dfrac{ w \cdot \ x }{ 2 } = \dfrac{ 53.3402 \cdot \ 22.0942 }{ 2 } \doteq 589.2557 \ \\ \ \\ S = 2 \cdot \ n \cdot \ S_{ 1 } = 2 \cdot \ 8 \cdot \ 589.2557 \doteq 9428.0904 \ \\ \ \\ V = \dfrac{ 1 }{ 3 } \cdot \ S \cdot \ v = \dfrac{ 1 }{ 3 } \cdot \ 9428.0904 \cdot \ 100 \doteq 314269.6805 = 314269.681$

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