The nth term of a geometric series has the form a*r^(n - 1) where a is the first term of the series and r is the common ratio.
Assume a number N is added to each term of the series. This gives the nth term as a*r^(n - 1) + N. This expression cannot be expressed in a form a'*r'^(n - 1). Therefore, the resulting series is not a geometric series.
The nth term of an arithmetic series is given by a + (n - 1)*d, where a is the first term and d is the common difference. If a number N is added to each term of the arithmetic series we get a + (n - 1)*d + N
We can write this as (a + N) + (n - 1)*d which is the nth term of an arithmetic series that has the first term as a + N and the common difference is d.
When a number is added to the terms of an arithmetic series we get a new arithmetic series.
Adding a number to the terms of a geometric series does not result in a geometric series but add a number to the terms of an arithmetic series results in a new arithmetic series.