Implementations of math function in Haskell

MathFunc.hs

--     | ln (abs(sin(x))), if x > 5
-- y = | x^2 + a^2,        if x <= 5 and a <= 3
--     | x / a + 7.8*a,    if x <= 5 and a > 3

y x a = if x > 5
        then log (abs (sin (x)))
        else
          if x <= 5 && a <= 3
          then x^2 + a^2
          else x / a + 7.8 * a

y' x a = case x > 5 of
  True  -> log (abs (sin (x)))
  False -> case x <= 5 && a <= 3 of
    True  -> x^2 + a^2
    False -> x / a + 7.8 * a

-- guards
y'' x a | x > 5            = log (abs (sin (x)))
        | x <= 5 && a <= 3 = x^2 + a^2
        | otherwise        = x / a + 7.8 * a