Verified answer

rappel : √(a²) = |a|

6 + 2√5 = 5 + 2√5 + 1 = (√5 + 1)²

a = |√5 + 1| = √5 + 1                             car √5 + 1 >0

6 - 2√5 = 5 -2√5 + 1 = (√5 - 1)²

b = |√5 - 1| = √5 - 1   car √5 > 1 et √5 - 1  > 0