`A =( 1 - 1/2 ) . ( 1 - 1/3 ) . ( 1 - 1/4 ) .... ( 1 - 1/2021 )`
`A = 1/2 . 2/3 . 3/4 .... 2020/2021`
`A = ( 1 . 2 . 3 .... 2020 )/( 2 . 3 . 4 .... 2021 )`
`A = 1/2021`
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`B = 1 1/2 . 1 1/3 . 1 1/4 . 1 1/5 .... 1 1/999`
`B = 3/2 . 4/3 . 5/4 . 6/5 .... 1000/999`
`B = ( 3 . 4 . 5 . 6 .... 1000 )/( 2 . 3 . 4 . 5 .... 999 )`
`B = 1000/2 = 500`
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`C = 1/15 + 1/35 + .... + 1/2499`
`C = 1/( 3 . 5 ) + 1/( 5 . 7 ) + .... + 1/( 49 . 51 )`
`C = ( 2/( 3 . 5 ) + 2/( 5 . 7 ) + .... + 2/( 49 . 51 ) ) . 1/2`
`C = ( 1/3 - 1/5 + 1/5 - 1/7 + .... + 1/49 - 1/51 ) . 1/2`
`C = ( 1/3 - 1/51 ) . 1/2`
`C = 16/51 . 1/2`
`C = 8/51`
