Đáp án: $A<2$
Giải thích các bước giải:
`A=\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+\frac{1}{5}+\frac{1}{6}+\frac{1}{7}+\frac{1}{8}+\frac{1}{9}`
`=(\frac{1}{2}+\frac{1}{3})+(\frac{1}{4}+\frac{1}{5})+(\frac{1}{6}+\frac{1}{7})+(\frac{1}{8}+\frac{1}{9})`
`<\frac{2+3}{2.3}+2.\frac{1}{4}+2.\frac{1}{6}+2.\frac{1}{8}`
`=\frac{5}{6}+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}`
`=\frac{5.2+6+4+3}{12}=\frac{23}{12}`
`<\frac{24}{12}=2`
`⇒A<2`
