The explicit formula help please

I have this really important test on it tomorrow and I need help with some studying... If you could explain how to get the answer to these 2 questions that would really help!

1) Find S23 of the Geometric series 1+(-2)+4+(-8)+...?
write the explicit formula before solving

2) What is the 18th term in the geometric sequence 1,2,4,8,...?
write the explicit formula before solving

For both questions, you need to find the ratio of the series and the sequence. To find it, simply divide the second term by the first term. It is always best to divide the third term by the second term just to make sure the ratio is the same.

1. -2/1 = -2. 4/-2 = -2. So the ratio, r, is equal to -2. In order to find sums of geometric series, use the equation Sn = a1((1 - r^n)/(1 - r)), where n is the sum you want to find and a1 is the first term of the series. In this case, n is equal to 23 and a1 is equal to 1. So, S23 = 1((1 - (-2)^23)/(1 - (-2)). S23 = 2,796,203.

2. 2/1 = 2. 4/2 = 2. So the ratio in this question is 2. In order to find a particular term, use the equation an = a1r^(n-1), where n is the term you want to find and a1 is the first term of the sequence. In this case, n is equal to 18 and a1 is equal to 1. So, a18 = 1(2)^(18-1). a18 = 131,072.