The derivative of a function g at a number a, denoted by g'(a), is given by the definition of the derivative:
In the definition of the derivative, "h" can be understood to be the horizontal change in the function with respect to a number <em>a</em>.
So g'(0) is
The variable in the limit is bound; without any other variables around, we can change the variable name to another reasonable variable name and it will still be the same. Hence we can change h to x and it will be equivalent: