SABCDE is a regular pentagonal pyramid.
ABCDE is regular pentagon. O is the centre of ABCDE
Triangle BOA
AB=40 cm,∠AOB=5360°=72°
OK⊥AB,AK=21AB=20 cm
OA=R=sin(2∠AOB)AK=sin(272°)20cm sin36°=410−25
OA=10−2580 cm 8. Right triangle SAO
H=SA2−OA2 Given SA=50 cm
H=(50)2−(10−2580)2 cm=
=10−2510186−505=105−593−255≈
≈36.636 cm
SABCDE=5(21)(OK)(AB)=25(AKtan36°)(AB)=
=45(AB)2tan36° sin36°=410−25
cos236°=1−sin236°=1−1610−25=166+25
tan236°=6+2510−25
tan36°=3+55−5
SABCDE=45(40cm)23+55−5
V=31SABCDEH=
=31(2000)3+55−5(105−593−255)cm3=
=3200003+593−255cm3≈
≈17745.29cm3≈1.8m3
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