Mik sửa lại đề nhé bn!
`B = 108/1 + 107/2 + 106/3 +...+ 2/107 + 1/108`
`= 1 + (107/2 + 1) + (106/3 + 1)+...+ (2/107 + 1) + (1/108 + 1)`
`= 1 + 109/2 + 109/3 +....+ 109/107 + 109/108`
`= 109/109 + 109/2 + 109/3 +...+ 109/107 + 109/108`
`= 109.(1/2 + 1/3 +....+ 1/107 + 1/108 + 1/109)`
`⇒ A/B = (1/2 + 1/3 + 1/4 +...+ 1/108 + 1/109)/(109.(1/2 + 1/3 +....+ 1/107 + 1/108 + 1/109)`
`= 1/109`