(1-1/21).(1-1/28).(1-1/36)...(1-1/1326)
=20/21 . 27/28 . 35/36 ... 1325/1326
=40/42 . 54/56 . 70/72 ... 2650/2652
=(5.8)/6.7 . (6.9)/7.8 . (7.10)/8.9 ... (50.53)/51.52
=(5.8)/6.7 . (6.9)/7.8 . (7.10)/8.9 ... (48.52)/50.51 . (50.53)/51.52
=(5.53)/7.51
=265/357