Android : Android Studio Error: "Cannot resolve symbol 'android'"

I'm new to Android development and I'm using Android Studio. After installing the latest SDK Tools revision in the SDK Manager, I reopened my project and received the following errors:

http://i.stack.imgur.com/mvsxV.png

For some reason, Android Studio shows a "Cannot resolve symbol 'android'" error even though it worked perfectly well before. How do I fix this?