Đáp án:
Giải thích các bước giải:
`B=1/2^2+1/3^2+1/4^2+1/5^2+1/6^2+1/7^2+1/8^2`
`=>B=1/2.2+1/3.3+1/4.4+1/5.5+1/6.6+1/7.7+1/8.8`
`=>B<1/1.2+1/2.3+1/3.4+1/4.5+1/5.6+1/6.7+1/7.8`
`=>B<1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8`
`=>B<1-1/8<1`
`=>B<1`
Vậy `B<1`.
`B=(1-1/2)*(1-1/3)*(1-1/4)*...*(1-1/20)`
`=>B=1/2*2/3*3/4*...*19/20`
`=>B=(1*2*3*...*19)/(2*3*4*...*20)`
`=>B=(1*cancel(2)*cancel(3)*...*cancel(19))/(cancel(2)*cancel(3)*cancel(4)*...*20)`
`=>B=1/20`
Vậy `B=1/20`
