Ta có:
`1/101>1/300`
`1/102>1/300`
`1/103>1/300`
..............................
`1/299>1/300`
`⇒\underbrace{\frac{1}{101}+\frac{1}{102}+\frac{1}{103}+..+\frac{1}{299}+\frac{1}{300}}_{200 phân số}``>1/300+1/300+1/300+...+1/300+1/300`
`⇒\underbrace{\frac{1}{101}+\frac{1}{102}+\frac{1}{103}+..+\frac{1}{299}+\frac{1}{300}}_{200 phân số}` `>1/300 .200=2/3`
`⇒B>2/3(đpcm)`