Exercise · ZW-SECOND-DERIVATIVE-TEST-007 · ★★☆ Intermediate
The function \(f(x,y)=x^{2} + 3 x y + y^{2}\) has a critical point at \((0,0)\). What does the second derivative test say?
Choose one option.
Show answer
- C. Saddle point
Explanation
\(f_{xx}=f_{yy}=2\) and \(f_{xy}=3\): \(D=4-9=-5<0\), saddle point. Even though \(f_{xx}>0\), the sign of \(D\) comes first.
Review the concept: Second derivative test
The second derivative test classifies a critical point of \(f\) as a minimum, maximum or saddle according to the sign of the discriminant \(D\) and of \(f_{xx}\) at that point.
\[D=f_{xx}f_{yy}-\left(f_{xy}\right)^{2}\]
More practice problems: second derivative test
- ZW-SECOND-DERIVATIVE-TEST-001 The function f(x,y)=x^2 + y^2 has a critical point at (0,0). What does the second derivative test s…
- ZW-SECOND-DERIVATIVE-TEST-002 The function f(x,y)=x^2 - y^2 has a critical point at (0,0). What does the second derivative test s…
- ZW-SECOND-DERIVATIVE-TEST-003 The function f(x,y)=- x^2 + 4 x - y^2 has a critical point at (2,0). What does the second derivativ…
- ZW-SECOND-DERIVATIVE-TEST-004 The function f(x,y)=x^3-3x+y^2 has critical points at (1,0) and (-1,0). What does the test say at (…
- ZW-SECOND-DERIVATIVE-TEST-005 The function f(x,y)=x^4+y^4 has a critical point at (0,0). What does the second derivative test say?
- ZW-SECOND-DERIVATIVE-TEST-006 For f(x,y)=x^2+xy+y^2, compute D=f_xxf_yy-f_xy^2 at the critical point (0,0).
- ZW-SECOND-DERIVATIVE-TEST-008 For f(x,y)=x^3+y^3-3xy, the point (1,1) is critical. What does the test say there?
- ZW-SECOND-DERIVATIVE-TEST-009 At a critical point of f we find D>0 and f_xx<0. What kind of point is it?
← ZW-SECOND-DERIVATIVE-TEST-006 ZW-SECOND-DERIVATIVE-TEST-008 →