设外弧长L1,内弧长L2外弧到内弧距离H,大半径R小半径(R-H)
L1=NπR/180
L2=Nπ(R-H)/180
两式作商
L1/L2=R/(R-H)
R =L1H/(L1-L2)
R-H=L1H/(L1-L2)-H=L2H/(L1-L2)
S扇形=1/2LR
外扇形面积-内扇形面积
=1/2L1R-1/2L2(R-H)
=1/2L1²H/(L1-L2)-1/2L2²H/(L1-L2)
=1/2(L1+L2)H
追问为什么要两式坐商?
追答作商便于推导出R,和R-H,(R和R-H要用已知的条件表示出来)