3/(x ² - 1) = 3/((x - 1) (x + 1)) = a/(x - 1) + b/(x + 1)
==> 3 = a (x + 1) + b (x - 1)
==> 3 = (a + b) x + a - b
==> a + b = 3 and a - b = 0
==> a = b
==> a + b = 2a = 3 ==> a = 2/3 and b = 2/3
So you have
3/(x ² - 1) = 2/(3(x - 1)) + 2/(3(x + 1))
