2. Consider the function . Find f'(x). (5 分)

6. What is the maximum directional derivative of the function f(x, y) = 75-2x² - y² at the point (5,5)? (5 分) Find a line tangent to the curve f(x, y) = 0 at this point. (5 分) Note:Point (a, b) means that the projections of the point on the x and y axes have coordinates a and b, respectively, in the rectangular coordinate system in two-dimensional space.