2.k can be negative.
3.y'=2x, the slope of the derivative function is equal to 2, and the inclination angle is equal to arctan2, which is a fixed value.
We say that the angle is steep, but we should pay attention to the fact that the scope of discussion is divided into two parts: acute angle and right angle.
1, table problem: a square table, cut off one corner, how many corners are left.
2, cut tofu problem: a piece of tofu cut three knives, can be cut