Question 3:

For this question, we should use both the definition of limit and the definition of derivative. For part a and b, we need to use the definition of derivative to get two equations. For part c, we should find the relationship between the previous equations and try to get a new equation to solve this problem. Firstly, we use the definition of limit to write the definition of equations that are in part a and b. Then, we can find there exists a relationship between these two equations, so we can get a new definition from that. Then we can use the new definition to write a new equation, so we just get the answer.