(2√7+√11)(√11-2√7) = (√11 + 2√7)(√11-2√7) = (√11)² - (2√7)² = 11 - 4*7 = 11-28 = -17 ∈ Z - целое число
(2√7+√11)(√11-2√7) = (√11 + 2√7)(√11-2√7) = (√11)² - (2√7)² = 11 - 4*7 = 11-28 = -17 ∈ Z - целое число