Đáp án:
B= (1/4-1)(1/9-1)...(1/100-1)
=(1/2²-1)(1/3²-1)...(1/10²-1)
=[(1/2-1)(1/2+1)](1/3-1)(1/3+1)]...[(1/10-1)(1/10+1)]
=-3/2².(-8/3²)....(-99/10²)
=-(1.3/2.2 . 2.4/3.3 ... 9.11/10.10)
=-(1.2...9/2.3...10 . 3.4...11/2.3...10)
=-(1/10.11/2)
=-11/20<-11/21( vì 11/20>11/21)
Vậy B<-11/21
XIN TLHN VÀ * NHÉ