Question

find the eighteenth term of the geometric sequence 1,-5,25,-125,....

Answer 1

@peachpi

Answer 2

i think here we using
an=a1+(n-1)d
right?

Answer 3

Here's where the formula comes in.
\[a _{n}=a _{1}r ^{n-1}\]

Answer 4

The one you posted is for arithmetic sequences

Answer 5

how can i know the different, and when do i have to use each one?

Answer 6

Well for this one, they said in the question that it was geometric :)

Answer 7

Usually they won't. You just have to practice at figuring out the pattern. If it's adding or subtracting use the arithmetic formula

Answer 8

If the pattern is multiplying use the geometric formula

Answer 9

oh ok thank u
so can u help me with taht question and do it with me step by step to understand it

Answer 10

yes. so we know the sequence is geometric. To use the formula we need to find a1 and r

Answer 11

a1 is the first term. What is it?

Answer 12

1

Answer 13

right.

Answer 14

r is the number that we multiply by to get to the next term. What do you multiply 1 by to get -5?

Answer 15

-5

Answer 16

right. So this is how we would write the formula for THIS sequence.
\[a _{n}=1(-5)^{n-1}\]

Answer 17

ok?

Answer 18

To find the 18th term, just plug in 18 for n