如图
h1/(h1+h)=r/R
解h1=hr/(R-r)
OB=根下（h1^2+r^2)=根下{[[hr/(R-r)]^2+r^2}
圆心角（弧度）＝2πr/OB=2πr/根下{[[hr/(R-r)]^2+r^2}