Company A plans inventory for n different SKU's. The characteristics of SKU i for i = 1, 2, ,n are: - annual demand, - purchase cost, - fixed ordering cost. Annual holding cost rate is the same for all SKU's, and it is r. Company A also has to pay the warehouse renting cost for the maximum inventory level that will be needed according to its ordering policy. The warehouse renting cost for SKU i for one year is . Perform all necessary calculations and write the formula for the optimal joint order interval when all items are ordered together.