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There are two locations it looks for this file: in more recent current versions of MATLAB, it will look in C:\Users\[...]\Documents\MATLAB. This is the preferred place to haev have the startup file. Another legacy However, it needs to be copied for every user.
Option for multi-user computer: Another location is in the MATLAB program files folder C:\Program FIles\MATLAB\[release]\toolbox\local. This does not need to be copied for each user, but it does need to be copied each time MATLAB is updated.
Editing the startup file
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