And the question is to determine the signs of the partial derivatives (a) fx (1,2) and (b) fy(1,2)
My thought was for (a), as I hold y constant and move positive on x, then what is the value of z. I'm not sure how I could really see that from this graph. At the point of x=1 the z graph looks flat....book says positive.
Then on (b), I was thinking hold x constant and move positive on the y axis...looks like z is neg there, which is what the book has.
Is there a better way to look at this? If I'm holding one of the variables constant, what do I do with the y value in (a) and with the x value in (b)?