Well, lets imagine that the gradient of f =\((x1,y1) at (1,2)\)
Since that, we can found the hence using this way : \((x1,y1) . (1,1)/sqrt(2) = 2 sqrt(2)\) with the result of : \(x1+y1 = 4\)
According to the 2nd directional derivative we can have \(-2 y1 =-6\)
Now it is not difficult to conclude that hence the gradient of f at (1,2) is (1,3) and we have to compute \((1,3) . (-1,-2)\) and divide by sqrt(5).
Finally, here is the answer that you will get in the end −(7/5√)
