设<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/33fa1ecc7caa4a4fad7b8db112cf404f.png?x-oss-process=image/sharpen,200"/>为椭圆<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/d5b02c5907304bdd90bc577904b7b1a5.png?x-oss-process=image/sharpen,200"/>按顺时针方向绕行一周，则曲线积分<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/d02f88f14b80490fb5d7d40a87fb6854.png?x-oss-process=image/sharpen,200"/>（） .