这样n°的圆心角所对的弧长的计算公式是L＝n*2πR/360
知道弧高和弦长求出半径和圆心角
R^2=(R-15)^2+75^2
R^2=R^2-30R+225+5625
30R=5850
R=175
圆心角n=2*arcsin(75/175)≈50°
L=50*2*3.14*175/360
= 152.638