~ Bạn tham khảo ~
` B = 1/(2^2) + 1/(3^2) + 1/(4^2) + .... + 1/(2019^2)`
`1/(2^2) < 1/(1.2) ; 1/(3^2) < 1/(2.3) ; 1/(4^2) < 1/(3.4) ; .... ; 1/(2019^2) < 1/(2018.2019)`
`=> B < 1/(1.2) + 1/(2.3) + 1/(3.4) + ..... + 1/(2018.2019)`
`=> B < 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/2018 -1/2019`
`=> B < 1 - 1/2019`
`=> B < 2018/2019` ( ĐPCM )
Vậy ` B < 2018/2019`
$#Vịt vàng$