How to get reciprocal (1/x) in scientific mode in Windows 10 calculator

The Windows 10 calculator changes slightly depending on the size of the window. You've probably got it a little bit on the small side, so, in order to access the reciprocal button, you need to use the shift button:

You'll notice when you click the shift button, you'll see a new set of functions. Alternatively, you can just make the window taller and they will all appear.

The standard mode has a reciprocal (1/x) button, but scientific mode does not

The Windows 10 calculator has some shortcuts you can use.

One of them is:

R - Select 1/x (Reciprocal).

Source Useful Calculator keyboard shortcuts in Windows 10