(g) The derivative of function f ( x,y) at the point (1,1) in the direction toward (2,1) is 3 and in the direction toward (0,0) is , then the derivative of f at (1,1) in the direction toward the point (4,5) is ________.

1. 填充題：共七題，每題 5 分，共 35 分。請將答案依題號順序寫在答案紙上，不必寫演算過程。