Could somebody please explain the method for answering a question like this?
It is easy to see, that the curve in definition of $A$ is an ellipse. Hence the set of extreme points of $A$ is this ellipse. Convexity is obvious: we use the same arguments as in the case of a closed ball.
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It is easy to see, that the curve in definition of $A$ is an ellipse. Hence the set of extreme points of $A$ is this ellipse. Convexity is obvious: we use the same arguments as in the case of a closed ball.