2 câu trả lời
`B=1/{20}+1/{30}+1/{42}+1/{56}+1/{72}+1/{90}+1/{110}`
`=1/{4.5}+1/{5.6}+1/{6.7}+1/{7.8}+1/{8.9}+1/{9.10}+1/{10.11}`
`=1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/{10}+1/{10}-1/{11}`
`=1/4-1/{11}`
`=7/{44}`
`B=1/20+1/30+1/42+1/56+1/72+1/90+1/110`
`=\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{8.9}+\frac{1}{9.10}+\frac{1}{10.11}`
`=1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/9+1/9-1/11/10-1/11`
`=1/4-1/11`
`=7/44`
