If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

## Algebra 1

### Unit 9: Lesson 3

Introduction to geometric sequences

# Using explicit formulas of geometric sequences

Sal finds the 5th term in the geometric sequence whose explicit formula is 3(-¼)^(i-1).

## Video transcript

- [Voiceover] The geometric sequence A sub I is defined by the formula and so they tell us that the Ith term is going to be equal to three times negative one fourth to the I minus one power. So given that, what is A sub five, the fifth term in the sequence? So pause the video and try to figure out what is A subscript five? Alright, well, we can just use this formula. A... A sub five is going to be... is everywhere I see an I or a place with a five is going to be equal to three times negative one fourth to the five minus one power. Well that's equal to three times negative one fourth to the fourth power. Well that's going to be equal to... lets see, we're raising it to an even power so it's going to give us a positive value since we're gonna be multiplying the negative an even number of times so it's gonna be a positive value so it's gonna be three times... let's see, one to the one fourth is- oh, one to the fourth power is just one, and then four to the fourth power... let's see, four squared is 16, so four squared times four squared is four to the fourth so it's 16 times 16 is 256. 256. And once again I know it's going to be positive because I'm multiplying a negative times itself four times, or I'm multiplying four negatives together, so that's going to give me a positive value. So I get three over 256. And we're done. That's the fifth term in our sequence. Positive three over 256.