theorem lean_workbook_plus_72909 : ∀ a b c : ℝ, 1 / 2 + 1 / 2 * Real.sqrt ((a * b + b * c + c * a) / (a ^ 2 + b ^ 2 + c ^ 2)) ≤ Real.sqrt (a ^ 2 / (4 * a ^ 2 + 5 * b * c)) + Real.sqrt (b ^ 2 / (5 * c * a + 4 * b ^ 2)) + Real.sqrt (c ^ 2 / (5 * a * b + 4 * c ^ 2)) := by sorry