A simple question.

[Zenith]

That isn't using infinity in a calculation, that is a formula that regresses "to infinity", or "tends to infinity". The value of infinity isn't used in that calculation since it doesn't exist.

If it is a value used, then please tell me what 10^infinity equals.

then it means this:

product(i=1,infinite) of 1/(10^i) i.e. 1/10 * 1/100 * 1/1000 * ...

should be valid as this:

sum (i = 1, infinity) of 1/10^i = 1/10 + 1/100 + 1/1000 + ...

is valid.