Step by step
The question
Question: a geometric sequence has a₃ = 18 and a₆ = 486.
Find the rule for term n.
1 Count the jumps
Term 3 is 18.
Term 6 is 486.
We don't know the terms between.
In a geometric sequence we multiply by the same number r every time.
How many jumps?
6 take away 3: 3 jumps.
Careful!
We count the jumps, not the terms.
4 terms have only 3 jumps between them.
2 Find r
Start at 18, multiply by r 3 times, and we get 486.
r times itself 3 times is r³.
To remove times 18, we divide by 18.
We do it on both sides.
Why both sides?
The = sign means both sides are the same amount.
If we change only one side, they are not the same any more.
18 divided by 18 is 1.
So just r³ is left.
486 divided by 18 is 27.
Which number times itself 3 times is 27?
3.
So r = 3.
3 Find the first term
a₁ is 2 jumps before a₃.
Going back, we divide by r each time.
3² is 9.
18 divided by 9 is 2.
4 Write the rule
The rule from the sequences lesson: the first term, times the ratio to the power (n − 1).
Put in a₁ = 2 and r = 3.
So the rule is aₙ = 2 × 3ⁿ⁻¹.
5 Check
Let's check with the terms we were given.
Put n = 3: the power is 2.
We get 18.
And n = 6 gives 486.
Both work!
Let's look back.
First: count the jumps.
Then: find r.
Then: find the first term.
Then: write the rule.
Last: check.