Suppose (a, b) is a critical point for $f(x, y)$. If $f_{xx}(a, b)f_{yy}(a, b) - (f_{xy}(a, b))^2 > 0$ and $f_{xx}(a, b) > 0$, then (a,b) is a ... local minimum local maximum saddle point ... no conclusion can be made about this critical point.
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Step 1: The given conditions are: $f_{xx}(a, b)f_{yy}(a, b) - (f_{xy}(a, b))^2 > 0$ $f_{xx}(a, b) > 0$ These are the conditions for the second derivative test for a function of two variables. Show more…
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If (a, b) is a critical point of a function f, and if fxx(a, b) < 0 and fyy(a, b) < 0, then what can you say about (a, b)? (i.e., is (a, b) a local max/min, saddle point, insufficient information?) For any value of fxy(a, b), what can you say about (a, b)? Show evidence to support your conclusion.
Madhur L.
Tricky but i will upvote
Hoan N.
Using the 2nd-Derivative Test given below, find the local extremas or saddle point of f(x, y) = x^3 + 3xy + y^3. THEOREM 11 Second Derivative Test for Local Extreme Values Suppose that f(x, y) and its first and second partial derivatives are continuous throughout a disk centered at (a, b) and that fx(a, b) = fy(a, b) = 0 . Then i. f has a local maximum at (a, b) if fxx < 0 and fxx fyy - fxy^2 > 0 at (a, b). ii. f has a local minimum at (a, b) if fxx > 0 and fxx fyy - fxy^2 > 0 at (a, b). iii. f has a saddle point at (a, b) if fxx fyy - fxy^2 < 0 at (a, b). iv. The test is inconclusive at (a, b) if fxx fyy - fxy^2 = 0 at (a, b). In this case, we must find some other way to determine the behavior of f at (a, b). It has saddle point at P(1,1) and local maximum at P(-1,-1) It has saddle point at P(0,0) and local minimum at P(-1,-1) It has saddle point at P(0,0) and local maximum at P(-1,-1) It has saddle point at P(0,0) and local minimum at P(-2,-2) It has saddle point at P(0,0) and local maximum at P(-2,-2) Leave blank
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