How to change the value of formula field by Salesforce Visualforce page?

Hi,

A formula is similar to an equation that is executed at run time. Depending on the context of the formula, it can make use of various data and operations to perform the calculation. A useful way to think about formulas is that they are similar to formulas on a spreadsheet. These formulas can make use of data and operations to calculate a new value of some type.

Formulas are widely used in the Force.com platform and should be considered first before jumping into Apex Code. The same formula syntax can be used to create default values, data validation rules, custom field formulas, conditional logic on Visualforce pages, and rules for determining when certain actions should occur, such as workflow actions.

for more details :- http://wiki.developerforce.com/page/An_Introduction_to_Formulas