Because the root (1-2a+a? )= root ( 1-a)？ , =| 1-a|, when a=3, 1-a < 0, so the root (1-2a+a? )=a- 1。 . So a+ root (1-2a+a? )=a+(a- 1)=2a- 1=5。 . So the first answer is wrong. The reason is that the absolute value is not removed correctly according to the number of roots.