已知,X²-3X+1 = 0 ,
则有：X²+1 = 3X ,
可得：X+1/X = 3 ；
因为,X²+1/X² = (X+1/X)²-2 = 3²-2 = 7 ,
所以,√(X²+1/X²-2) = √(7-2) = √5 .