Press Ctrl+Enter to
copy a single entry to selected cells. If, for example, you want to add numbers
in B1:B5 to numbers in A1:A5, you could do it like this:

Select
the range C1:C5.

Type
the formula =A1+B1.

Press Ctrl+Enter.
Excel enters the formula in all the cells of the range.

Another example.
Suppose you wanted to enter the same formula in seven separate cells. Just
select the cells, type the formula, and press Ctrl+Enter. Excel enters the
formula in all seven cells, whether they are adjacent to each other or not.