玄的一半对应的圆心角的正弦值为0.4/1.128=0.35460992907801418439716312056738,所以圆心角=20.769539734610503342903396525682度.所以整个玄长对应的圆心角应乘以2,=41.539079469221006685806793051365
所以弧长=2*3.14*1.128/360*41.539079469221006685806793051365=0.81779289922362722062590167992473