# Sequence of sum

1. Jan 31, 2013

### Biosyn

1. The problem statement, all variables and given/known data

Find the sum of $5^1-5^2+5^3-5^4+...-5^{98}$

a. (5/4)(1-5^99)
b. (1/6)(1-5^99)
c. (6/5)(1+5^98)
d. (1-5^100)
e. (5/6)(1-5^98)

2. Relevant equations

3. The attempt at a solution

I feel as though this is actually a simple problem and that I'm not looking at it the right way.

[$5^1 + 5^3 + 5^5....5^{97}$] + [$-5^2-5^4-5^6...-5^{98}$]

Last edited: Jan 31, 2013
2. Jan 31, 2013

### jbunniii

Do you know how to sum $x^n$ in general? What is $x$ here?

3. Jan 31, 2013

### Biosyn

$x$ will be 5?

$$\sum_{i=0}^{48} (5^{2i + 1})$$ + $$\sum_{i=0}^{49} (5^{2i})$$

Never mind, I figured it out!

Last edited: Jan 31, 2013
4. Jan 31, 2013

### jbunniii

Actually, it looks to me like
$$-\sum_{n=1}^{98}(-5)^n$$

5. Feb 1, 2013

### Biosyn

I used Sn = $\frac{a_1*(1-r^n)}{1-r}$

Sn = $\frac{5*(1-(-5)^98)}{1-(-5)}$

= $\frac{5*(1-(-5)^98)}{6}$

= (5/6)*(1-(-5)^98)