We are given a sequence $a_n$ such that $a_1 = 2$ and $a_{n+1} = 3a_n + 4$. We need to find the general term $a_n$.

AlgebraSequences and SeriesRecurrence RelationsGeometric SequencesAlgebraic Manipulation
2025/3/11

1. Problem Description

We are given a sequence ana_n such that a1=2a_1 = 2 and an+1=3an+4a_{n+1} = 3a_n + 4. We need to find the general term ana_n.

2. Solution Steps

First, let's rewrite the recurrence relation as
an+1+x=3(an+x)a_{n+1} + x = 3(a_n + x).
an+1=3an+2xa_{n+1} = 3a_n + 2x.
Comparing with the original relation an+1=3an+4a_{n+1} = 3a_n + 4, we have 2x=42x = 4, which gives x=2x = 2.
Therefore, an+1+2=3(an+2)a_{n+1} + 2 = 3(a_n + 2).
Let bn=an+2b_n = a_n + 2. Then bn+1=3bnb_{n+1} = 3b_n.
This means that the sequence bnb_n is a geometric sequence with common ratio

3. Since $b_1 = a_1 + 2 = 2 + 2 = 4$, we have $b_n = b_1 \cdot 3^{n-1} = 4 \cdot 3^{n-1}$.

Therefore, an+2=43n1a_n + 2 = 4 \cdot 3^{n-1}, which gives an=43n12a_n = 4 \cdot 3^{n-1} - 2.

3. Final Answer

an=43n12a_n = 4 \cdot 3^{n-1} - 2

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