Question

Write the recursive formula for 5, 8, 11, ...
an = an-1 + 3
an = an-1 - 3
an = 3an-1
an = an-1 + 1

i dont undderstand the a,n formula at all

Answer 1

@geerky42

Answer 2

In your sequence, \(a_1=5\) is the first term.

In the fourth of the given answer choices, for example, you have
\[\begin{align*}a_n=a_{n-1}+1~~\implies~~a_2&=a_{2-1}+1\\&=a_1+1\\&=5+1\\&=6\end{align*}\]
But in your sequence, the second term is \(a_2=8\neq6\), so it can't be this sequence.

Given some term in the sequence, the recursive formula tells you what the next term will be. The value of the next term depends on the previous term.