A geometric sequence works like this: you have a starting point [tex]a_1[/tex], and you generate the next term by multiplying the previous one by the common ratio [tex]r[/tex]
In your case, the common ratio is -1, which means that you get every other term by changing the sign of the previous one.
So, the sequence goes like this:
[tex]10,\ -10,\ 10,\ -10,\ldots[/tex]
