4,
A=4/3.7+4/7.11+4/11.15+...+4/103.107
=1/3-1/7+1/7-1/11+1/11-1/15+...+1/103-1/107
=1/3-1/107=104/321<1
3,x-4/y-3=4/3
=>3x-12=4y-12
=>3x-4y=0
=>3(x-y)-y=0=>15-y=0=>y=15
=>x=20
5,
Ta có : 2004A=2004^2004+2004/2004^2004+1
=1+2004/2004^2004+1
2004B=2004^2005+2004/2004^2005+1
=1+2004/2004^2005+1
Vì 2004/2004^2004+1>2004/2004^2005+1
=>A>B
