數學題...

設石柱直徑 = D cm
繩長 + 1.2 cm = 4 × 石柱圓周, 而圓周 = π × 直徑 = π × D
250 cm + 1.2 cm = 4 × π × D
251.2 cm = 4 × π × D
D = 251.2 cm ÷ 4 ÷π = (251.2 ÷π) cm
如π = 3.14, 石柱直徑 = (251.2 ÷ 3.14) cm = 20 cm

繩長:250+1.2=251.2cm
圍繞圓柱體石柱1次的繩長:251.2/4=62.8cm=圓周
石柱的直徑是:62.8/3.14=20cm

設直徑是r cm
4( π r) = 2.5 + 1.2
4π r = 3.7
π r = 0.925
r = 0.294436644
所以直徑是0.294cm (準至小數點後兩個位)

2.5m=250cm
250cm+1.2cm=251.2cm
4*2*π*R=251.2cm
r=~10cm

Diameter = 2*10cm = 20cm

方程式:
2 x 圓同率 x 半徑 = 圓周

2 x (3.14) x 半徑 = 4 x (2.5 + 1.2)
半徑大約等於 2.36cm (3個有效數字)

(2.5+1.2)除4

係「25cm+1.2cm/4」啩!