Let f be a differentiable real function defined in (a, b). Prove that f is convex if and only if f' is monotonically increasing. Assume next that f"(x) exists for every x e (a, b), and prove that f is convex if and only if f"(x)20 for all x e (a, b).

Advanced Engineering Mathematics
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ISBN:9780470458365
Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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14. Let f be a differentiable real function defined in (a, b). Prove that f is convex if
and only if f' is monotonically increasing. Assume next that f"(x) exists for
every x € (a, b), and prove that f is convex if and only if f"(x) 20 for all x e (a, b).
Transcribed Image Text:14. Let f be a differentiable real function defined in (a, b). Prove that f is convex if and only if f' is monotonically increasing. Assume next that f"(x) exists for every x € (a, b), and prove that f is convex if and only if f"(x) 20 for all x e (a, b).
Expert Solution
Step 1

A function f is convex in a domain a,b if for all 0λ1fλx1+(1-λ)x2λfx1+1-λfx2  x1,x2a,b.

Also if a function is monotonically incresing then x1x2 implies that fx1fx2.

The derivative of a function exists then it can be expressed using the limit definitions.

Now a differentiable function is convex if it graph lies above all of its tangents.

Also, it is known that if s<y<x<t, then  ft-fxt-xft-fst-sfy-fsy-s.

The theorem can be proved using all these results.

 

 

 

 

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