设<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/125fd395f1d540afbaf9c190f8f741c6.png?x-oss-process=image/sharpen,200"/>是抛物面<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/974173b151cd4522a77294797f66cda5.png?x-oss-process=image/sharpen,200"/> 取下侧，则曲面积分<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/a87cb39d8804495fb375f5098f5056db.png?x-oss-process=image/sharpen,200"/>（）.