How to find the length of a diagonal of a polygon - PSAT Math

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Question

Regular Octagon has sidelength 1.

Give the length of diagonal .

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Answer

The trick is to construct segments perpendicular to from and , calling the points of intersection and respectively.

Octagon_1

Each interior angle of a regular octagon measures

,

and by symmetry, ,

so .

This makes and triangles.

Since their hypotenuses are sides of the octagon with length 1, then their legs - in particular, and - have length .

Also, since a rectangle was formed when the perpendiculars were drawn, .

The length of diagonal is

.

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