$a$) `{x+1}/{x-5} - {1-x}/{x+5} -{2x(1-x)}/{25-x^2}` ($ĐKXĐ: x \neq ±5$)
`= {(x+1)(x+5)}/{(x-5)(x+5)} - {(1-x)(x-5)}/{(x-5)(x+5)} - {2x(1-x)}/{(5+x)(5-x)}`
`= {x^2 + 6x + 5 - (-x^2 + 6x-5)}/{(x-5)(x+5)} + {2x(1-x)}/{(x+5)(x-5)}`
`= {x^2 + 6x + 5 +x^2 - 6x + 5 + 2x - 2x^2}/{(x+5)(x-5)}`
`= {2x+10}/{(x+5)(x-5)}`
`= 2/{x-5}`.
$b$) `1/{x+3} - 1/{x-3} - {2x}/{x^2-9}` ($ĐKXĐ: x \neq ±3$)
`= {x-3}/{(x+3)(x-3) - {x+3}/{(x-3)(x+3)} - {2x}/{(x-3)(x+3)}`
`= {(x-3)-(x+3)-2x}/{(x-3)(x+3)}`
`= {x-3-x-3-2x}/{(x-3)(x+3)}`
`= {-2x-6}/{(x-3)(x+3)}`
`= -2/{x-3}`.
