Question

The third term in a sequence is 11.

The term-to-term rule is "take away 4".
Write an expression, in terms of n, for the nth term of the sequence.
The third number is 12​

Answer 1

Since the person hasn't specified I'll solve for both f(3) = 12 and f(3) = 11.

f(n) = 20 - 4(fn-1) is the formula for when third term is 12

f(n) = 21 -4(n-1) when f(3) = 11

Explanation:

Okay since we know d = -4, we can start to make an explicit rule.

But first, we need the first term.

If f(3) = 12, we know f(1) = 20

So.

f(n) = 20 - 4(n-1) when f(3) = 12

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For f(3) = 11, we know f(1) = 21

So.

f(n) = 21 -4(n-1) when f(3) = 11