Ta có S=`1/(2^2)`+`1/(3^2)`+`1/(4^2)`+...+`1/(9^2)` <`1/(2^2)`+`1/(2.3)`+`1/(3.4)`+....+`1/(8.9)` =`1/(2^2)`+`1/2`-`1/3`+`1/3`-`1/4`+....+`1/8`-`1/9` =`1/4`+`1/2`-`1/9`=`23/36`<`32/36`=`8/9`
Ta có S=`1/(2^2)`+`1/(3^2)`+`1/(4^2)`+...+`1/(9^2)` <`1/(2^2)`+`1/(2.3)`+`1/(3.4)`+....+`1/(8.9)` =`1/(2^2)`+`1/2`-`1/3`+`1/3`-`1/4`+....+`1/8`-`1/9` =`1/4`+`1/2`-`1/9`=`23/36`<`32/36`
