❶ 计算弧长
设半径为r,弧所对的圆心角为a°(r-10)²+50²=r²r=130cosa=(130²+130²-100²)/2*130*130=0.70414a=45.24弧长=圆周长*a/360=2*3.14*130*45.24/360=102.6cm