‌设函数<img src="https://storage.tutusouti.com/psst/files/d5c0ea86571b413ea36c05cb4424fe23.jpeg?x-oss-process=image/sharpen,200"/>，为了使<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/aa4dc038832747e789d6122f090703fd.png?x-oss-process=image/sharpen,200"/>在<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/4f23939dd15c4f99bc27eeb274f1a715.png?x-oss-process=image/sharpen,200"/>处可导，问：<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/963051583118453d869e2b5782fcacf8.png?x-oss-process=image/sharpen,200"/>应取的值为（）.