1. (a) Let f be defined for all (x, y) by f(x, y) = xy² and f(0,0) = 0. Show that f(x, y) and f(x, y) exist for all (x, y). (b) Show that f has a directional derivative in every direction at every point. (c) Show thatf is not continuous at (0, 0). (Hint: Consider the behaviour of f along the curve x = y2.) Is f differentiable at (0, 0)?

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
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al xm matrix. By the mean value theorem (Theorem 2.1.2), there exists a z in [x, y] such that
g(y) – 8 (x) = Vg(z) · (y – x) = b · (f'(z)(y – x)). The Cauchy-Schwarz inequality, (1.1.38), then
implies |g(y) - 8 (x) < ||b| · |f'(z)V- )|| = ||f'(z)(y – x)||, and (3) follows.
By applying the inequality (3) to the new function y f(y) -f'(w)y, with w a fixed vector, the
following inequality is obtained for all x, y, and w in R":
|f(y) – f(x) – f'(w)(y – x)|| < max |(f'(z) – f'(w))(y - x)||
(4)
[A'x]Əz
PROBLEMS FOR SECTION 2.9
1. (a) Let f be defined for all (x, y) by f(x, y) =
xy2
and f (0, 0) = 0. Show that f{(x, y)
and f;(x, y) exist for all (x, y).
(b) Show thatf has a directional derivative in every direction at every point.
(c) Show that f is not continuous at (0, 0). (Hint: Consider the behaviour of f along the curve
x = y2.) Is f differentiable at (0, 0)?
Existence and Uniqueness of Solutions of
Systems of Equations
2.10
This section is concerned with the svstem of n eauations in n unknowns, of the form
10:18 P
4/3/202
BA
delete
home
Or
pua
os ud
->
backspace
wnu
lock
8.
6
home
Transcribed Image Text:al xm matrix. By the mean value theorem (Theorem 2.1.2), there exists a z in [x, y] such that g(y) – 8 (x) = Vg(z) · (y – x) = b · (f'(z)(y – x)). The Cauchy-Schwarz inequality, (1.1.38), then implies |g(y) - 8 (x) < ||b| · |f'(z)V- )|| = ||f'(z)(y – x)||, and (3) follows. By applying the inequality (3) to the new function y f(y) -f'(w)y, with w a fixed vector, the following inequality is obtained for all x, y, and w in R": |f(y) – f(x) – f'(w)(y – x)|| < max |(f'(z) – f'(w))(y - x)|| (4) [A'x]Əz PROBLEMS FOR SECTION 2.9 1. (a) Let f be defined for all (x, y) by f(x, y) = xy2 and f (0, 0) = 0. Show that f{(x, y) and f;(x, y) exist for all (x, y). (b) Show thatf has a directional derivative in every direction at every point. (c) Show that f is not continuous at (0, 0). (Hint: Consider the behaviour of f along the curve x = y2.) Is f differentiable at (0, 0)? Existence and Uniqueness of Solutions of Systems of Equations 2.10 This section is concerned with the svstem of n eauations in n unknowns, of the form 10:18 P 4/3/202 BA delete home Or pua os ud -> backspace wnu lock 8. 6 home
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