Suppose $f(x,y) = (x-y)(4-xy).$ Answer the following. Each answer should be a list of points $(a,b,c)$ separated by commas, or, if there are no points, the answer should be NONE.

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1. Find the local maxima of $f.$
Answer:

2. Find the local minima of $f.$
Answer:

3. Find the saddle points of $f.$
Answer:

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