`C=( 1 - 1/3 )( 1 - 1/6 )( 1 - 1/10 )( 1 - 1/15 )...(1 - 1/210)`
`C= ( 3/3 - 1/3 )( 6/6 - 1/6 )( 10/10 - 1/10 )(15/15 - 1/15 )...(210/210 - 1/210)`
`C= 2/3 . 5/6 . 9/10 . 14/15 . ... . 209/210`
`C= ( 2 . 2 )/( 3 . 2 ) . ( 5 . 2 )/( 6 . 2 ) . ( 9 . 2 )/( 10 . 2 ) . ... . ( 209 . 2 )/( 210 . 2 )`
`C= 4/6 . 10/12 . 18/20 . ... . 418/420`
`C= ( 1 . 4 )/( 2 . 3 ) . ( 2 . 5 )/( 3 . 4 ) . ( 3 . 6 )/( 4 . 5 ) . ... . ( 19 . 22 )/( 20 . 21 )`
`C= ( 1 . 4 . 2 . 5 . 3 . 6 . ... . 19 . 22 )/( 2 . 3 . 3 . 4 . 4 . 5 . ... . 20 . 21 )`
`C= ( ( 1 . 2 . 3 . ... . 19 ) . ( 4 . 5 . 6 . ... . 22 ))/(( 2 . 3 . 4 . ... . 20) . ( 3 . 4 . 5 . ... . 21 ))`
`C= ( 1 . 22 )/( 20 . 3 )`
`C= 22/60`
`C = 11/30`
Vậy `C= 11/30`
