Ta có:
`A= 13/25+9/10-11/15+13/21-15/28+17/36-...+197/4851-199/4950 `
`A=13/25+[(9/10-11/15)+(13/21-15/28)+(17/36-19/45)+...+(197/4851-199/4950)]`
`A=13/25+[(27/30-22/30)+(52/84-45/84)+(85/180-76/180)+...+(9850/242550-9751/242550)]`
`A=13/25+[1/6+1/12+1/20+...+1/2450]`
`A=13/25+[1/2.3+1/3.4+1/4.5+...+1/49.50]`
`A=13/15+[1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+...+1/49-1/50]`
`A=13/15+[1-1/50]`
`A=13/15+[50/50-1/50]`
`A=13/15+49/50`
`A=130/150+147/150`
`A=277/150>9/10`
Vậy `A>9/10`
$#Rabifoot$