Đáp án:
`A>9/10`
Giải thích các bước giải:
`A=13/25+9/10-11/15+13/21-15/28+17/36-...+197/4851-199/4950`
`=>A/2=13/2.25+9/2.10-11/2.15+13/2.21-15/2.28+17/2.36-...+197/2.4851-199/2.4950`
`=>A/2=13/50+9/20-11/30+13/42-15/56+17/72-...+197/9702-199/9900`
`=>A/2=13/50+9/4.5-11/5.6+13/6.7-15/7.8+17/8.9-...+197/98.99-199/99.100`
`=>A/2=13/50+((1/4+1/5)-(1/5+1/6)+(1/6+1/7)-(1/7+1/8)+(1/8+1/9)-...+(1/98+1/99)-(1/99+1/100))`
`=>A/2=13/50+(1/4-1/100)`
`=>A/2=13/50+12/50`
`=>A/2=1/2`
`=>A=1>9/10`
Vậy `A>9/10`.
