Posted by **Katie** on Wednesday, March 23, 2011 at 6:09pm.

what is the length of any diagonal of a regular pentagon with a side of 20mm ?

- Geometry quick i have test tomorrow -
**Katie**, Wednesday, March 23, 2011 at 7:15pm
what is the length of any diagonal of a regular pentagon with a side of 20mm

- geometry -
**Reiny**, Wednesday, March 23, 2011 at 7:24pm
first find the size of each interior angle

angle = 180(5-2)/5 = 108°

let the length of the diagonal be x

by cosine law

x^2 = 20^2 + 20^2 -2(20)(20)cos 108°

I am sure you can finish it from there.

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