Đáp án:
`A = 75/14`
Giải thích các bước giải:
`7/6 + 13/12 + 21/20 + 31/30 + 43/42`
` = (6+1)/6 + (12 + 1)/12 + (20+1)/20 + (30 +1)/30 + (42+ 1)/42`
` = 6/6 + 1/6 + 12/12 +1/12 + 20/20 + 1/20 + 30/30 + 1/30 + 42/42 + 1/42`
` = 1 + 1/6 + 1 + 1/12 + 1 + 1/20 + 1 + 1/30 + 1 + 1/42`
` = (1 + 1 + 1 + 1 + 1) + (1/6 + 1/12 +1/20 + 1/30 +1/42)`
` = 1 . 5 + (1/(2.3) + 1/(3.4) + 1/(4.5) + 1/(5.6) + 1/(6.7))`
` = 5 + (1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7)`
` = 5 + (1/2 - 1/7)`
` = 70/14 + 7/14 - 2/14`
` = 75/14`
Vậy `A = 75/14`
