Practice for classifying critical points

A function f(x,y)f(x, y)is found to have the following critical points (listed in the first column). The second derivative test function, D, is defined as:


D(x,y)=3x22y9xy2x2yD(x, y) = \dfrac{3x^22y -9xy}{2x-2y}

and the second derivative with respect to x is given as:

fxx=3yxxyf_{xx} = \dfrac{3y-x}{x-y}
Classify each as a local minima, maxima, or saddle point
(-11, 1)
(-11, -9)
(7, 2)
(3, -4)
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