why did they build the pentagon in that shape
The Pentagon The site originally chosen was Arlington Farms which had a roughly pentagonal shape, so the building was planned accordingly as an irregular pentagon.[15] Concerned that the new building could obstruct the view of Washington, D.C., from Arlington Cemetery, President Roosevelt ended up selecting the Hoover Airport site instead.[16] The building retained its pentagonal layout because a major redesign at that stage would have been costly, and Roosevelt liked the design. Freed of the constraints of the asymmetric Arlington Farms site, it was modified into a regular pentagon which resembled the star forts of the gunpowder age.[17][18]