Tolong bantuannya........

Luas juring = α/(360°) luas lingkaran
80 = α/(360°) . πr^2
80 = α/(360°) . πr^2
Misal α/360° = x
80 = x . πr^2
x = 80/(πr^2)

Keliling juring = d + α/(360°) . Keliling lingkaran ====> d = diameter
K = 2r + x . 2πr
K = 2r + 80/(πr^2) . 2πr
K = 2r + 160/r
K = 2r + 160r^-1
Agar diperoleh keliling minimum kita turunkan dan sama dg nol
K' = 2 - 160r^-2 = 0
=> -160/r^2 = -2
=> -160 = -2r^2
=> r^2 = 80
=> r = √80 = 4√5