~ Ban tham khảo ~
`([x-1]/[x+1]-[x+1]/[x-1]):1/[x-1]`
`=([(x-1)(x-1)]/[x^2-1]-[(x+1)(x+1)]/[x^2-1]):1/[x-1]`
`=[(x-1)(x-1)-(x+1)(x+1)]/[x^2-1]:1/[x-1]`
`=[(x^2-2x+1)-(x^2+2x+1)]/[x^2-1]*x-1`
`=[x^2-2x+1-x^2-2x-1]/[x^2-1]*x-1`
`=[-4x(x-1)]/[x^2-1]`
`=[-4x(x-1)]/[(x-1)(x+1)]`
`=[-4x]/[x+1]`