What is Mean value theorem ?

If f is a differentiable function on the interval [a, b], then there exists a number c between a and b such that:

1) f' ' (c) = [ f (b) - f (a) ] / (b-a)

or, equivalently,

2) f(b) - f (a) = f' ' (c) (b-a)

For example, if a car travels 180 km from 1pm to 3pm, its average velocity is 90km/ hr.
Then the speedometer must have read 90km/hr at least once.
Note: 1pm and 3pm represent a and b on the above formula respectively.