题目

Bruce Horowitz writes in USA Today that Domino’s Pizza is changing its pizza recipe . It will be a completely new pizza with a different crust, cheese , and sauce. Some observers say that the company is taking a big risk with such a total reform of its main product. However, in a 2009 consumer survey , Domino’s tied for last place for taste . Domino’s was voted the best for convenience and price. This change is not something that Domino’s took lightly. They spent two years testing many different recipes before settling on the new Domino’s Pizza.. The company will have an advertising 公差为1的等差数列{an}满足a2+a4+a6=9，则a5+a7+a9的值等于    ．