Tính B=(1/4 - 1 ) . (1/9 - 1 ) . (1/16 - 1 ) . .... (1/81 -1) . ( 1/100 -1 )

`B=(1/4-1)(1/9-1)(1/16-1)....(1/81-1)(1/100-1)`

`B=(-3)/4.(-8)/9.(-15)/16....(-80)/81.(-99)/100`

`-B=(3/4).(8/9).(15/16)....(80/81).(99/100)`

`-B=(1.3)/(2.2).(2.4)/(3.3).(3.5)/(4.4)...(8.10)/(9.9).(9.11)/(10.10)`

`-B=(1.3.2.4.3.5....8.10.9.11)/(2.2.3.3.4.4....9.9.10.10)`

`-B=(1.cancel{3}.cancel{2}.cancel{4}.cancel{3}.cancel{5}....cancel{8}.cancel{10}.cancel{9}.11)/(cancel{2}.2.cancel{3}.cancel{3}.cancel{4}.cancel{4}....cancel{9}.cancel{9}.cancel{10}.10)`

`-B=(1.11)/(2.10)=11/20`

`B=(-11)/20`