Bài `6:`
`a) (199...9)/(999...5)` (`10` chữ số `9` ở tử, `10` chữ số `9` ở mẫu)
` = (1999...9 : 1999...9)/(999...5 : 199..9)` (cùng chia cho tử số)
` = 1/5`
`b) (121212)/(424242)`
` = (121212 : 60606) / (424242 : 60606)`
` = 2/7`
`c) (187187187) / (221221221)`
` = (187187187 : 17017017) / (221221221 : 17017017)`
` = 11/13`
`d) (3.7.13.37. 39 - 10101)/(505050 + 70707)`
` =( 39. 10101 - 10101 ) (50.10101 + 7.10101)`
` = (10101 . 38)/ (10101 .57)`
` = 38/57= 2/3`
Bài `7:`
`a) C= 3/(4.7) + 3/ 7.10 + 3/10.13 +...+3/ 73.76`
` = 1/4 - 1/7 + 1/7 - 1/10 + 1/10 - 1/13 + ...+ 1/73 - 1/76`
` = 1/4 - 1/76`
`9/38`
`b) D = 4/(3.7) + 4/(7.11) + 4/(11.15) + ...+ 4/(107.111)`
` = 1/4 -1/7 + 1/7 - 1/11 + 1/11 - 1/15 +...+ 1/107 - 1/111`
` = 1/4 - 1/111`
`= 107/444`
`c) B = 2/15 + 2/35 + 2/63 +...+ 2/399`
` = 2/(3.5) + 2/(5.7) + 2/(7.9) + ....+2/(19.21)`
` = 1/3 - 1/5 + 1/5 - 1/7 +1/7-1/9 +...+ 1/19 - 1/21`
` = 1/3 - 1/21`
` = 2/7`
