9. (a) Suppose that {ad is a decreasing sequence of real numbers. Prove that if 2::'1 ak

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9.

(a) Suppose that {ad is a decreasing sequence of real numbers. Prove that if 2::'1 ak converges, then kak ~ 0 as k ~ 00.

(b) Let Sn = 2:~=1 (_1)k+1 /k for n E N. Prove that S2n is strictly increasing, S2n+1 is strictly decreasing, and S2n+1 - S2n ~ 0 as n ~ 00.

(c) Prove that part

(a) is false if "decreasing" is removed.

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