Đáp án:
`A<1/2`
Giải thích các bước giải:
đề lỗi ở `12/256` thành `1/256`
ta có : `A=1/4+1/16+1/36+1/64+1/100+1/144+1/196+12/256+1/324<1/2`
`A=1/4+1/16+1/36+1/64+1/100+1/144+1/196+12/256+1/324`
`=1/2^2+1/4^2+1/6^2+1/8^2+1/10^2+1/12^2+1/14^2+1/16^2+1/18^2`
do :
`1/2^2<1/1.3`
`1/4^2<1/3.5`
`1/6^2<1/5.7`
`. . . . . `
`1/18^2<1/17.19`
______________________________________________________
`=>` `A<1/1.3+1/3.5+1/5.7+...+1/17.19`
`=>` `1/2×(1-1/3+1/3-1/5+1/5-1/7+...+1/17-1/19)`
`=>` `1/2×(1-1/19)`
`=>` `1/2×18/19`
`=>` `9/19`
do `9/19` `<` `1/2`
nên `A<1/2`
