`A=1/21+1/22+1/23+...+1/50`
`⇒A=(1/21+1/22+...+1/30)+(1/31+1/32+...+1/40)+(1/41+1/42+...+1/50)`
`⇒A>(\underbrace{1/30+1/30+...+1/30}_{\text{10 số hạng}})+(\underbrace{1/40+1/40+...+1/40}_{\text{10 số hạng}})+(\underbrace{1/50+1/50+...+1/50}_{\text{10 số hạng}})`
`⇒A>10. 1/30+10. 1/40+10. 1/50`
`⇒A>10/30+10/40+10/50`
`⇒A>1/3+1/4+1/5`
`⇒A>{20+15+12}/60`
`⇒A>47/60>45/60=3/4`
`⇒A>B`