Cho A = 1/31 + 1/32 + 1/33 +...+ 1/60. Chứng minh rằng
Giải thích
Ta có
A=131+132+133+...+160=131+132+....+144+145+146+147+....+159+160
Mà 131+132+....+144+145>145+145+....+145+145=1545=13
146+147+....+159+160>160+160+....+160+160=1560=14
Nên A=131+132+....+144+145+146+147+....+159+160>13+14=712
Vậy A>712