Đáp án:
`2A=2.( 1/(1.3) + 1/(3.5) + 1/(5.7) + 1/(7.9) + 1/(9.11))`
`2A=2/(3.5)+2/(5.7)+2/(7.9)+2/(9.11)`
`2A=1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11`
`2A=1/3-1/11=8/33`
`A=8/33:2=4/33`
Vậy A=`4/33`
`B= 1/2 + 1/(2^2)+ 1/(2^3) +...+ 1/(2^100)`
`2B=1+1/2+1/(2^2)+....+1/(2^99)`
`2B-B=(1+1/2+1/(2^2)+....+1/(2^99))-( 1/2 + 1/(2^2)+ 1/(2^3) +...+ 1/(2^100))`
`B=1+1/2+1/(2^2)+.........+1/(2^99)-1/2-1/(2^2)-................-1/(2^100)`
`B=1-1/(2^100)=(2^100-1)/(2^100)`
Vậy `B=(2^100-1)/(2^100)`
XIN HAY NHẤT NHA
