请解释cost of goods sold公式：

看公式是个merchandising business：beginning inventory 是原本的库存 ＋ cost of goods purchased 采购的新货 ＝ cost of goods available for sale 总共的库存 cost of goods available for sale 总共的库存 －co...谢谢。但原公式是：beginning inventory + cost of goods pruchased =cost of goods sold除非假设 ending inventory＝0, 也就是说卖出了所有的库存。要不然 beginning inventory + cost of goods pruchased 不等于 cost of goods sold。