Hướng dẫn trả lời:
`A=2/1.2+2/2.3+...+2/99.100.`
`=2.(1/1.2+1/2.3+...+1/99.100).`
`=2.(1-1/2+1/2-1/3+...+1/99-1/100).`
`=2.(1-1/100).`
`=2.(100/100-1/100).`
`=2. 99/100.`
`=99/50.`
`B=6/15.18+6/18.21+...+6/87.90.`
`=2.(3/15.18+3/18.21+...+3/87.90).`
`=2.(1/15.18+1/18.21+...+1/87.90).`
`=2.(1/15-1/18+1/18-1/21+...+1/87-1/90).`
`=2.(1/15-1/90).`
`=2.(6/90-1/90).`
`=2. 5/90.`
`=1/9.`