3: Proof followings: (a) If the length (norm, absolute) of a vector function A(u) is a constant, A' will be perpen- dicula to A. ◆ Hint: the absolut of a vector A is given by √A A. The question said that this absolute value is a constant. (b) If the direction of a vector function A(u) is a constant, A' will be parallel to A. ♣ Hint: The vector function said by the equestion is expressed as A(u) by na arbitrary scalar function f(u) and a constant vector e.