老師：android studio版本1.1.0 JRE 版本1.8.0_25，提示gradle project sync failed,Basic functionality(e.g.diting. debugging)will not work properly.最下面提示Failed to find:com.android.support:appcompat-v7:21.+, ?Install Repository and sync project . show in File. show in project Structure dialog 請問如何解決，謝謝！