1. Consider the paraboloid f(x,y) = ax² + bry + cy², where we will assume that a, c# 0. We will investigate the behavior of f at its critical point. (a) Show that (0,0) is the only critical point when b² – 4ac # 0. (b) By completing the square, show that 2 4ас — 6? b x + 2a f(x, y) = 4a2 (c) Suppose that D= 4ac – 6². Without using the second derivative test: i. Suppose that D > 0 and a > 0. Show that f has a local minimum at (0,0). [Hint: Show that f(0,0) = 0. Use the fact that a and D are both positive to conclude that when x,y # 0, f(x, y) > 0.] ii. Suppose that D> 0 and a < 0. Show that ƒ has a local maximum at (0, 0). iii. Finally, suppose that D < 0. Show that f has a saddle point. [Hint: Explain why the tangent plane at (0 0) has equation z = 0 We wish to show that f crosses this tangent plane by showwing there

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Chapter2: Second-order Linear Odes
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1. Consider the paraboloid f(x,y) = ax² + bxy + cy2, where we will assume that a, c 0. We will investigate the
behavior of f at its critical point.
(a) Show that (0,0) is the only critical point when b2
4ас + 0.
(b) By completing the square, show that
2
4ас
62
f (x, y)
= a
2a
4а2
(c) Suppose that D=
4ac – b2. Without using the second derivative test:
i. Suppose that D > 0 and a > 0. Show that f has a local minimum at (0,0).
f(0,0) = 0. Use the fact that a and D are both positive to conclude that when x, y + 0, f(x, y) > 0.]
ii. Suppose that D >0 and a < 0. Show that f has a local maximum at (0,0).
iii. Finally, suppose that D < 0. Show that f has a saddle point. [Hint: Explain why the tangent plane
at (0,0) has equation z = 0. We wish to show that f crosses this tangent plane, by showing there
exist different paths for which f has opposite signs along those paths.]
[Hint: Show that
(d) Point out that the value D as described above is exactly the function D(x, y) involved in the second
derivative test, and that the results match that test.
Transcribed Image Text:1. Consider the paraboloid f(x,y) = ax² + bxy + cy2, where we will assume that a, c 0. We will investigate the behavior of f at its critical point. (a) Show that (0,0) is the only critical point when b2 4ас + 0. (b) By completing the square, show that 2 4ас 62 f (x, y) = a 2a 4а2 (c) Suppose that D= 4ac – b2. Without using the second derivative test: i. Suppose that D > 0 and a > 0. Show that f has a local minimum at (0,0). f(0,0) = 0. Use the fact that a and D are both positive to conclude that when x, y + 0, f(x, y) > 0.] ii. Suppose that D >0 and a < 0. Show that f has a local maximum at (0,0). iii. Finally, suppose that D < 0. Show that f has a saddle point. [Hint: Explain why the tangent plane at (0,0) has equation z = 0. We wish to show that f crosses this tangent plane, by showing there exist different paths for which f has opposite signs along those paths.] [Hint: Show that (d) Point out that the value D as described above is exactly the function D(x, y) involved in the second derivative test, and that the results match that test.
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