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I understand that for both problems I need to pove that are subsets of S and C for some positive number t. But, in the first one, I don't know how to get the y involve.
For the second one, I have w in D(x,a), then |w-y| = |w-x+x-y| < |w-x| + |x-y| = |w-x| + 1, but then... how do I get it < 1?