‎设L是曲线<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/d75095123370458c980dd4ade8558f1f.png?x-oss-process=image/sharpen,200"/>与直线y=x所围区域在第一象限的整个边界曲线，f(x,y)是连续函数，则<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/3037229ed908430cbea420ffc8f0ac04.png?x-oss-process=image/sharpen,200"/>‏