Let f be defined on [a, b], if f has a local maximum at a point x E (a, b), and if f'(x) exists, then f'(x) = 0. The statement for local minimum point is also true.

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
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PROOF THE STATEMENT FOR LOCAL MINIMUM POINT IS ALSO TRUE PLEASE ???

Theorem :
Let f be defined on [a, b] , if f has a local maximum at a
point x E (a, b), and if f'(x) exists, then f'(x) = 0.
The statement for local minimum point is also true.
Proof :
Let 8 > 0,
a <x - 8 < x < x+ 8<b
If
x - 8<t<x
then
f(t) < f(x),t– x < 0
f(t) - f(x)
t-x
f(t) - f(x)
lim
t-x
: f'(x) 2 0
(1)
If
x <t< x+8 then
f(t) < f(x),
t- x>0
f(t) - f(x)
<0
t-x
f(t)-f(x)
lim
t-x
: f'(x) <0
: f'(x) < 0..
(2)
Hence from (1) and (2 ) we conclude that f'(x) = 0
Transcribed Image Text:Theorem : Let f be defined on [a, b] , if f has a local maximum at a point x E (a, b), and if f'(x) exists, then f'(x) = 0. The statement for local minimum point is also true. Proof : Let 8 > 0, a <x - 8 < x < x+ 8<b If x - 8<t<x then f(t) < f(x),t– x < 0 f(t) - f(x) t-x f(t) - f(x) lim t-x : f'(x) 2 0 (1) If x <t< x+8 then f(t) < f(x), t- x>0 f(t) - f(x) <0 t-x f(t)-f(x) lim t-x : f'(x) <0 : f'(x) < 0.. (2) Hence from (1) and (2 ) we conclude that f'(x) = 0
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