设函数<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/e64bc4c38a994ece86adbaeb5c5838cc.png?x-oss-process=image/sharpen,200"/>在点<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/2845cf41546e4cd4835e0201e38cd28f.png?x-oss-process=image/sharpen,200"/>处存在关于<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/b2cf6bc3d359467f8ef41b774a5c43dc.png?x-oss-process=image/sharpen,200"/>和<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/03f9a2d28037473385f0c5a7a8889816.png?x-oss-process=image/sharpen,200"/>的一阶偏导数，则极限<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/2622e1febdde40a0adf7fe1b90d2e326.png?x-oss-process=image/sharpen,200"/>的值为( ).