Bài 5 đề 1
Ta có :
196+197/197+198
=196/197+198+197/197+198
Mà 196/197+198<196/197
197/197+198< 197/198
<=> 196/197+197/198>196+197/197+198
<=>a<b
Bài 5 đề 2
B=1/4+1/5+1/6+.....+1/19
=1/4+(1/5+1/6+...+1/9)+(1/10+1/11+....+1/19)
Vì 1/5+1/6+..+1/9>1/9+1/9+.....+1/9
=5/9>1/2
Vì 1/10+1/11+.....+1/19>1/19+1/19+.....+1/19
=10/9>1/2
<=>B=1/4+5/9+10/19>1/4+1/2+1/2>1
B>1
