help! algebra 2!
1. 2+4+8+... to 20 terms
a. -1,048,575
b. -2,097,150
c. 2,097,150
d. 3,145,725

2. Find a_1 for the geometric series in which S_n=4,882,812, r=-5, and n=10
a. -2
b. -3
c. 2
d. 3

*I got C for both questions, i am not sure if i am right though

Question

help! algebra 2!
1. 2+4+8+... to 20 terms
a. -1,048,575
b. -2,097,150
c. 2,097,150
d. 3,145,725

2. Find a_1 for the geometric series in which S_n=4,882,812, r=-5, and n=10
a. -2
b. -3
c. 2
d. 3

*I got C for both questions, i am not sure if i am right though

anonymous · Answer

$$
2+ 4+ 8 + \cdots +2^{20}\

$$

anonymous · Answer

$$
\sum_{i=1}^n a^i=\frac{a
   \left(1-a^{20}\right)}{1-a}
$$

anonymous · Answer

Take a=2, and n=20 compute the answer

anonymous · Answer

$$
\frac{2
   \left(1-2^{20}\right)}{1-2}=2,097,150
$$

anonymous · Answer

ok what more do you need help with?

anonymous · Answer

I was wondering if the 2nd answer was right.