Nice to see, good way of learning. All commented etc... Nice
by the way let me tell you something if you don't mind. You could use different names on your variables, because when you have a program with a few vars, it is harder for other and for you to understand the code...
Let's say like:
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int detRadius;
int valRadius;
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I guess it looks better
Another thing that you probably already know is the << endl; that you could be using instead of "\n". ("\n" is fine, just giving you more options)
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cout << "Welcome to Circumference Calculator." << end; //Title
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But after all it looks good, and thanks for sharing. Keep up