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Q3 (Prove that Σ 17 < oo if r > 1) . Let f : (0,00) → R be a twice differentiable function with f) 2 0 for all r E (0, 0o) (

Q3 (Prove that P∞ k=1 1/kr < ∞ if r > 1) . Let f : (0,∞) → R be a twice differentiable function with f ''(x) ≥ 0 for all x ∈ (0,∞).

(a) Show that f '(k) ≤ f(k + 1) − f(k) ≤ f '(k + 1) for all k ∈ N.

(b) Use (a), show that Xn−1 k=1 f '(k) ≤ f(n) − f(1) ≤ Xn k=2 f '(k).

(c) Let r > 1. By finding a suitable function f to (b), show that 1/nr − Xn k=1 1/kr ≤ 1 (r − 1)nr−1 − 1 /r − 1 ≤ 1 − Xn k=1 1/kr for all positive integer n.

(d) Show that if r > 1, limn→∞ Pn k=1 1/kr exists.

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Q3 (Prove that P∞ k=1 1/kr < ∞ if r > 1) . Let f : (0,∞) → R be a twice differentiable function with f ''(x) ≥ 0 for all x ∈ (0,∞). (a) Show that f '(k) ≤ f(k + 1) − f(k) ≤ f '(...
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