`(1-1/3)(1-1/6)(1-1/10)(1-1/15)...(1-1/780)`
`=2/3 . 5/6 . 9/10 . 14/15 ... 779/780`
`=4/6 . 10/12 . 18/20 ...1558/1560`
`=(1.4)/(2.3) . (2.5)/(3.4) . (3.6)/(4.5) ... (38.41)/(39.40)`
`=(1.4.2.5.3.6...38.41)/(2.3.3.4.4.5...39.40)`
`=((1.2.3...38).(4.5.6...41))/((2.3.4...39).(3.4.5...40))`
`=(1.2.3..38)/(3.4.5...40) . (4.5.6...41)/(2.3.4...39)`
`=2/(39.40) . (40.41)/(2.3)`
`=41/297`