Đáp án:
`A = 1+1/5+3/5.7+3/7.9+...+3/35.37+3/37.39`
` = 1+ 1/5 + 3/2 . ( 2/5.7+2/7.9+...+2/35.37+2/37.39)`
` = 1 + 1/5 + 3/2 . (1/5 - 1/7 + 1/7 - 1/9 + .... + 1/35 - 1/37 + 1/37 - 1/39)`
` = 1+1/5 + 3/2 . (1/5 - 1/39)`
` = 6/5 + 3/2 . 35/195`
` = 6/5 + 17/65`
` = 19/13`
Vậy ` A = 19/13`
`Go od luck!`