數學知識交流[001]

圖片參考：http://i1099.photobucket.com/albums/g395/jasoncube/4GTOYNt_zps9dcac0bc.jpg


圖片參考：http://i1099.photobucket.com/albums/g395/jasoncube/672A547D540D-5_zps16b42fc1.png

http://s1099.photobucket.com/user/jasoncube/media/4GTOYNt_zps9dcac0bc.jpg.html?sort=3&o=1

http://i1099.photobucket.com/albums/g395/jasoncube/672A547D540D-5_zps16b42fc1.png

圖片參考：http://imgcld.yimg.com/8/n/HA00869018/o/20130607180202.jpg
Interior angle of regular pentagon= (5-2)(180o)/5= 108o Angle OAB= 108o/2= 54o Intriangle OAB,1/AB =sin54oAB =1.24 units (corr. to 2 sig. fig.) 1/OA =tan54oOA =0.73 unit (corr. to 2 sig. fig.) Let F bea point on OB such that OF is perpendicular to DC. Angle FBC= 108o-(180o-90o-54o)= 72o Intriangle FBC,FC/1.24= sin72oFC =1.18 units (corr. to 2 sig. fig.) Area ofpentagon ABCDE= Areaof AEB + Area of EBCD= (EO+OB)(OA)/2+ (EO+OB+DC)(FC)/2= (1+1)(0.73)/2+ (1+1+1.24)(1.18)/2= 2.63sq. units

http://i.imgur.com/4GTOYNt.jpg

Find the area of the regular pentagon ABCDE

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