Theo mình thì
A=1/3+1/6+1/10+...+1/561
A=1/1*3+1/2*3+1/2*5+...+1/17*33
1/2*A=1/1*3*2+1/2*3*2+1/2*5*2+...+1/17*33*2
1/2*A=1/2*3+1/3*4+1/4*5+...+1/33*34
1/2*A=1/2-1/3+1/3-1/4+1/4-1/5+...+1/33-1/34
1/2*A=1/2-1/34
1/2*A=8/17
A=8/17 chia cho 1/2
A=16/17
Vì 16/17>8/9
=> A>8/9