How to find a description of a calc function fast by its name?

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Does not the online help do this for you? Press F1 to open the help, and search for the function you want.

Or, click on the Function Wizard (fx) to the left of the data entry box.

Or, from the menu Insert | Function will also bring up the function wizard.

If you are new to LibreOffice, you may find the online LibreOffice Help useful. There is a list of functions there also.

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