- #1
zhuyilun
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Homework Statement
1.if the derivative of f(x,y) with respect to x and y both exist, then f is differentiable at (a,b)
2. if (2,1) is a critical point of f and fxx (2,1)* fyy (2,1) < (fxy (2,1))^2, then f has a saddle point at (1,2)
3. if f(x,y) has two local maxima, then f must have a local minimun
4. Dk f(x,y,z)= fz (x,y,z)
Homework Equations
The Attempt at a Solution
1. i think it is right, but i can't come up with a good explanation
2. i think it is right according to the second derivative test, but for some reason, the wording of this question keeps making me think this question is wrong
3. i thnk it is right, because if f( x,y) has 2 local max, then there must be a local min between those two local max because the graph must decrease after the first local max
4. i have no idea, can someone explain?
can someone tell me what i did is right or nor?