`B = 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56`
`B = 1/( 1 xx 2 ) + 1/( 2 xx 3 ) + 1/( 3 xx 4 ) + 1/( 4 xx 5 ) + 1/( 5 xx 6 ) + 1/( 6 xx 7 ) + 1/( 7 xx 8 )`
`B = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8`
`B = 1 - 1/8`
`B = 7/8`
Vậy `, B = 7/8 .`