nomer 12 ..bantu ya kak

keliling lingkaran = 2.phi.r
72/360.x 2phi.r = 31,4
1/5 x 2.3,14.r = 31,4
1/5 x 2r x 3,14 = 31,4
(2/5)r = 10
r = 10 x 5/2
r = 25
Luas juring = 72/360.phi.r^2
= 1/5 x 3,14 x 25^2
= 3,14/5 x 625
= 3,14 x 125
= 392,5 cm^2