Ta thấy:
1/2x2<1/1x2
1/3x3<1/2x3
1/4x4<1/3x4
....
1/200x200<1/199x200
Ta có:Q<1/1x2+1/2x3+1/3x4+...+1/199x200
1/1x2+1/2x3+1/3x4+...+1/199x200
=1-1/2+1/2_1/3+1/3-1/4+...+1/199-1/200
=1-1/200
=199/200
=>Q<199/200
3/4=150/200
Mà 199/200>150/200
Vậy Q<3/4
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