In (1), the result is a piecewise function, and the whole function is divided into three sections, OA, AB and BC. Each segment can be solved by the analytical formula of the corresponding function and then substituted into the coordinate values, but if the BC segment is a parabola, it may not be solved because of insufficient conditions.

(2) The problem belongs to the scope of derivative solution. The derivative can be obtained by segmentation.

(3) Solve the problem on the basis of problem (2). When the derivative of the function is 0, the corresponding point gets the extreme value, which can be obtained first, and then the maximum value or minimum value can be obtained by comparison.