(1/41+1/42+1/43+...+1/50)+(1/51+1/52+...+1/100)
1/41+1/42+...+1/50 > 1/50+1/50+...+1/50 (10 số hạng)
=1+1+...+1/50=10/50=1/5
1/51+1/52+...+1/100 > 1/100+1/100+1/100 (50 số hạng)
=1+1+...+1/100=50/100=1/2
=> 1/41+1/42+1/43+...+1/99+1/100> 1/5 +1/2=7/10