50m=50000mm,8.3cm=83mm
83π+(83+2*0.2)π+(83+4*0.2)π+......+(83+2n*0.2)π=50000
83+83+0.4+83+2*0.4+......+83+n*0.4=50000/π
83(n+1)+0.4+2*0.4+......+n*0.4=50000/π
83(n+1)+0.4n(n+1)/2=50000/π
83n+83+0.2n(n+1)≈15915.4943
83n+0.2n²+0.2n=15832.4943