“底面”的英语？

袋子平整折起后，底面与一侧交叠。When the bag is folded flat, the bag bottom overlaps one side of the bag.也可能是为了使油漆有个适宜底面.Alternatively it may be solely a method of forming a suitable background for painting.同高的平行六面体(的体积)之比等于其底面(积)之比.Par...