We are given a sequence $\{a_n\}$ defined by the recurrence relation $a_{n+1} = 3a_n + 4$ with the initial condition $a_1 = 2$. We need to find the general term $a_n$ of this sequence.

AlgebraSequencesRecurrence RelationsGeometric SequencesAlgebraic Manipulation
2025/3/11

1. Problem Description

We are given a sequence {an}\{a_n\} defined by the recurrence relation an+1=3an+4a_{n+1} = 3a_n + 4 with the initial condition a1=2a_1 = 2. We need to find the general term ana_n of this sequence.

2. Solution Steps

First, we try to find a constant cc such that an+1+c=3(an+c)a_{n+1} + c = 3(a_n + c). This gives us an+1=3an+2ca_{n+1} = 3a_n + 2c. Comparing this with the given recurrence relation an+1=3an+4a_{n+1} = 3a_n + 4, we have 2c=42c = 4, which implies c=2c = 2.
Now, let bn=an+2b_n = a_n + 2. Then, bn+1=an+1+2=3an+4+2=3an+6=3(an+2)=3bnb_{n+1} = a_{n+1} + 2 = 3a_n + 4 + 2 = 3a_n + 6 = 3(a_n + 2) = 3b_n. This means that the sequence {bn}\{b_n\} is a geometric sequence with common ratio

3. Also, $b_1 = a_1 + 2 = 2 + 2 = 4$.

Therefore, bn=b13n1=43n1b_n = b_1 \cdot 3^{n-1} = 4 \cdot 3^{n-1}.
Since bn=an+2b_n = a_n + 2, we have an=bn2=43n12a_n = b_n - 2 = 4 \cdot 3^{n-1} - 2.

3. Final Answer

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

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