Có `1^2-1=a^2-a+a-1=a(a-1)+(a-1)=(a-1)(a+1)`

`=>2^2-1=(2-1)(2+1)=1*3`

tương tự `3^2-1=2*4`

`4^2-1=3*5`

`.....`

`2025^2-1=2024*2026`

`=>(1/2^2-1)*(1/3^2-1)*.....*(1/2025^2-1)`

`=(-1)*(1-1/2^2)*(1-1/3^2)*....*(1-1/2025^2)`

`=(-1)*(2^2-1)/2^2*(3^2-1)/3^2*......*(2025^2-1)/2025^2`

`=(-1)*(1*3)/2^2*(2*4)/3^2*....*(2024*2026)/2025^2`

`=(-1)*(1*2*3*.........*2024)/(2*3*.....*2025)*(3*4*5*.....*2026)/(2*3*.....*2025)`

`=(-1)*1/2025*2026/2`

`=(-2026)/4050=(-1013)/2025`