# Finding tangent plane with the max range value and z value being equal

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Rahal Rodrigo on 16 Apr 2021 at 15:17
Edited: Andrew Newell on 16 Apr 2021 at 22:00
Hi there. I'm plotting a tangent plane to a particular surface. However, the range for both x and y if +- sqrt(2), which is +-1.41. The expected value of z for the tangent plane is sqrt(2) as well. However, when i run the command, MATLAB produces an error for that particulat range (produces sucessful output on ranges such as +-2). Would be grateful if someone provided insight what I could do to solve this issue. The code is shown below. Any advice would be greatly apreciated.

Andrew Newell on 16 Apr 2021 at 21:57
Edited: Andrew Newell on 16 Apr 2021 at 22:00
Notice that j is empty, then try this:
X = -1.41:0.01:1.41;
[~,idx] = min(abs(X-1));
disp(X(idx)-1)
This being floating point, you're not getting exact increments of . You could scale everything so x, y are and then find and . Or you could do it like this:
z = @(x,y) sqrt(4-x.^2-y.^2);
X = -1.41:0.01:1.41;
[x,y] = meshgrid(X);
[~,j] = min(abs(X-1));
fx0 = fx(j,j);
fy0 = fy(j,j);
z1 = @(x,y) z(1,1) + fx0*(x-1) + fy0*(y-1);
As a side note, you don't need . You can use j directly as logical indexing.