ll AT&T ? 12:06 AM coursesite.lehigh.edu iii. Use (i) and (ii) to show that dy dz = -1. 2 of 2 2. UNDERSTANDING SADDLES AND CONCAVITY. Suppose f(r, y) is differentiable, so that at the point P(2, -4), f(2, -4) = 17, fr(2, -4) = 4, fyy (2, -4) = 2 and fay(2, -4) = 3. Let v = (3, –1) and w = (3, 4). (a) Consider the trace of the graph z = f(x, y) at y = -4. Is the curve you get concave up or down at the point (2, -4, 17)? How do you know? (b) If at a point P we have Vf (P) - w = 0 critical point of f. and Vf (P) - v = 0, show that P must be a (c) Since at the point P above we have frr = 4, fyy = 2 and fry = 3 which are all positive, is the graph z = f(x,y) concave up at (2, -4, 17)? Explain. (d) The previous part means that if you trace the graph with a vertical plane in some direction, the concavity will be down. To see this, let's consider values of f along a line through P, say r(t) give a direction vector (a, b) for the line. Let F(t) = f(x(t),y(t)) f (2 + at, -4 + bt). Then by the Chain Rule 2+ at and y(t) = -4 + bt, where a and b are constants which + b12tat.-4+bt) |(2+at,-4+bt) Show that = 4a? + 262 + 6ab. Find an example of (a, b) for which this is negative! This tells you how to move away from P in the domain to go down along the graph of f.

Algebra & Trigonometry with Analytic Geometry
13th Edition
ISBN:9781133382119
Author:Swokowski
Publisher:Swokowski
Chapter11: Topics From Analytic Geometry
Section11.4: Plane Curves And Parametric Equations
Problem 33E
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Question 2 part a

ll AT&T ?
12:06 AM
coursesite.lehigh.edu
iii. Use (i) and (ii) to show that dy dz
= -1.
2 of 2
2. UNDERSTANDING SADDLES AND CONCAVITY. Suppose f(r, y) is differentiable, so that at
the point P(2, -4), f(2, -4) = 17, fr(2, -4) = 4, fyy (2, -4) = 2 and fay(2, -4) = 3. Let
v = (3, –1) and w = (3, 4).
(a) Consider the trace of the graph z = f(x, y) at y = -4. Is the curve you get concave up
or down at the point (2, -4, 17)? How do you know?
(b) If at a point P we have Vf (P) - w = 0
critical point of f.
and Vf (P) - v = 0, show that P must be a
(c) Since at the point P above we have frr = 4, fyy = 2 and fry = 3 which are all positive,
is the graph z = f(x,y) concave up at (2, -4, 17)? Explain.
(d) The previous part means that if you trace the graph with a vertical plane in some
direction, the concavity will be down. To see this, let's consider values of f along a line
through P, say r(t)
give a direction vector (a, b) for the line. Let F(t) = f(x(t),y(t)) f (2 + at, -4 + bt).
Then by the Chain Rule
2+ at and y(t) = -4 + bt, where a and b are constants which
+ b12tat.-4+bt)
|(2+at,-4+bt)
Show that
= 4a? + 262 + 6ab.
Find an example of (a, b) for which this is negative! This tells you how to move away
from P in the domain to go down along the graph of f.
Transcribed Image Text:ll AT&T ? 12:06 AM coursesite.lehigh.edu iii. Use (i) and (ii) to show that dy dz = -1. 2 of 2 2. UNDERSTANDING SADDLES AND CONCAVITY. Suppose f(r, y) is differentiable, so that at the point P(2, -4), f(2, -4) = 17, fr(2, -4) = 4, fyy (2, -4) = 2 and fay(2, -4) = 3. Let v = (3, –1) and w = (3, 4). (a) Consider the trace of the graph z = f(x, y) at y = -4. Is the curve you get concave up or down at the point (2, -4, 17)? How do you know? (b) If at a point P we have Vf (P) - w = 0 critical point of f. and Vf (P) - v = 0, show that P must be a (c) Since at the point P above we have frr = 4, fyy = 2 and fry = 3 which are all positive, is the graph z = f(x,y) concave up at (2, -4, 17)? Explain. (d) The previous part means that if you trace the graph with a vertical plane in some direction, the concavity will be down. To see this, let's consider values of f along a line through P, say r(t) give a direction vector (a, b) for the line. Let F(t) = f(x(t),y(t)) f (2 + at, -4 + bt). Then by the Chain Rule 2+ at and y(t) = -4 + bt, where a and b are constants which + b12tat.-4+bt) |(2+at,-4+bt) Show that = 4a? + 262 + 6ab. Find an example of (a, b) for which this is negative! This tells you how to move away from P in the domain to go down along the graph of f.
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