As we know, a rollUpSummary field can be created on the master object of a master-detail relationship. the relationship b/w account and opportunity are look up, then how it is possible to create a roll-up summary field on account related to opportunity ?
This feature was delivered in Spring 08 based on this Idea:
The recently announced Winter 08 feature - Roll-up Summary Fields for Standard Objects (Opp-Opp Line in Winter 08) - is another great step forward. While this will extend the Roll-Up Summary Field functionality to work with Opportunities and their Line Items, it would be tremendously useful to extend that same functionality to the remaining standard objects and existing relationships. For example... Example 1 [COMING IN SPRING 08] Create a Roll-up Summary field on the Account object that calculates the SUM of all related Opportunities where Stage = "Closed/Won." This is an obvious win for organizations that wish to easily identify the value of a particular customer in terms of sold business. Example 2 [SEPARATE IDEA POSTED] Create a Roll-up Summary field on the Account object that calculates the COUNT of all related Cases where Type or Status = X. Use your imagination but this could be an easy way for support organizations to identify how many cases a customer has submitted versus their paid support agreement. Example 3 [SEPARATE IDEA POSTED] Create a Roll-up Summary field on the Account or Contact object that calculates the COUNT of all related Tasks where Type = "Call" Status = "Complete." This would allow organizations that focus on a high volume of customer interaction to easily evaluate those trends.