Đáp án:
`A=3/5`
`E=1/9`
`F=9/25`
Giải thích các bước giải:
`A=7/10.11+7/11.12+7/12.13+...+7/69.70`
`A=7[1/10-1/11+1/11-1/12+1/12-1/3+...+1/69-1/70]`
`A=7[1/10-(1/11-1/11+1/12-1/12+1/3-...-1/69+1/70)]`
`A=7[1/10-1/70]`
`A=7 . 3/35`
`A=3/5`
___________________________
`E=6/15.18+6/18.21+6/21.24+...+6/87.90`
`E=2[1/15-1/18+1/18-1/21+1/21-1/24+...+1/87-1/90]`
`E=2[1/15-(1/18-1/18+1/21-1/21+1/24-...-1/87+1/90)]`
`E=2[1/15-1/90]`
`E=2 . 1/18`
`E=1/9`
_____________________________
`F=3^2/8.11+3^2/11.14+3^2/14.17+...+3^2/197.200`
`F=9/8.11+9/11.14+9/14.17+...+9/197.200`
`F=3[1/8-1/11+1/11-1/14+1/14-1/17+...+1/197-1/200]`
`F=3[1/8-(1/11-1/11+1/14-1/14+1/17-...-1/197+1/200)]`
`F=3[1/8-1/200]`
`F=3 . 3/25`
`F=9/25`
