5. (12 points) Let $A$ be an $m \times n$ matrix and $x$ be an $n \times 1$ vector.

(a) (4 points) Given $\|x\|=1$, what is the least square solution for $Ax=0$?

(b) (8 points) Please prove your answer.

(Please leave it blank if you don't know the correct answer, or you will get at most minus 5 points (until questions 3, 4, and 5 are 0 points) for the wrong answer. 不會寫請留白，答錯最多倒扣5分，扣至3, 4, 5題0分為止。)