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iOS Objective-C Basics (Retired) Introduction to Objective-C Introducing @property

Error while Running with the new changes

So I did the changes exactly as in the video, yet I get these compiling errors:

-Use of undeclared identifier '_radius'; did you mean 'radius'? -Use of undeclared identifier '_radius' -Use of undeclared identifier '_center'; did you mean 'center'? -Use of undeclared identifier '_center'