How do I set up two cells with a reciprocal formula?

[New user]

I would like to be able to input a number into Cell 1 and obtain a
calculation in Cell 2 via a formula and equally input a number into Cell 2
and obtain the reciprocal in Cell 1 via the same formula whilst guarding the
formula.

Very many thanks in advance

 

[Bill Renaud]

Put a value in Cell1.
Put the formula in Cell2.
Putting a value in Cell1 works as normal.
To force Cell2 to a value, select Cell2, but then use Tools|Goal Seek to
set the value of Cell2 to the desired value by changing Cell1.