Đáp án:
`81/10`
Giải thích các bước giải:
`1/2+5/6+11/12+19/20+29/30+41/42+55/56+71/72+89/90 `
`=1-1/2+1-1/6+1-1/12+1-1/20+1-1/30+1-1/43+1-1/56+1-1/72+1-1/90 `
`=9-(1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90) `
`=9- (1/(1.2)+1/(2.3)+1/(3.4)+1/(4.5)+1/(5.6)+1/(6.7)+1/(7.8)+1/(8.9)+1/(9.10)) `
`=9-(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+1/8-1/9+1/9-1/10) `
`=9 - (1 - 1/10) = 9 - 9/10 = 81/10`