a² a a² a(a - 1) a² - a² + a a
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a² - 1 a + 1 (a - 1)(a + 1) (a - 1)(a + 1) (a + 1)(a - 1) a² - 1
a²/(a² - 1) - a/(a + 1) = a²/(a - 1)(a + 1) - a/(a + 1) =
a²/(a - 1)(a + 1) - a(a - 1)/(a - 1)(a + 1) = (a² - a² + a)/(a - 1)(a + 1) = a/(a² - 1)