一條中二數..唔識...解到20點..只有一日

A = πR^2(x/ 360)---(1)
L = 2πR(x/360)--(2)

from (2),
L = 2πR(x/360)
R=L(360)/2πx--(3)

sub (3) into (1),

A=π(R)(R)(x/ 360)
A=π[L(360)/2πx]R(x/ 360)
A=LR/2
R=2A/L//

A=πR ×R ×﹙ｘ/360﹚ L=πR ×2 ×﹙ｘ/360﹚ A/L=πR ×R ×﹙ｘ/360﹚/πR ×2 ×﹙ｘ/360﹚ A/L=R/2 So. R=2A/L