设f(u)具有二阶连续导数，而<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/e2fa598808ee413ba764cc62b6af2b15.png?x-oss-process=image/sharpen,200"/>满足方程<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/0e0bf6d41002445dbed3c1b94728f0d5.png?x-oss-process=image/sharpen,200"/>,则f(u)=（）.