设<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/02981380d36040d1b14ed9456435fbe6.png?x-oss-process=image/sharpen,200"/>由曲面<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/c507c344f9b64ba282c79bece01752db.png?x-oss-process=image/sharpen,200"/>与三坐标平面所围成的闭区域，则<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/cfec3777354747eca6f43d72767cbdc6.png?x-oss-process=image/sharpen,200"/>（）．