We are given a recursive sequence defined by $a_1 = 2$ and $a_{n+1} = 3a_n + 4$. We want to find the general term $a_n$ of the sequence.

AlgebraSequences and SeriesRecursive SequencesGeometric Sequences
2025/4/24

1. Problem Description

We are given a recursive sequence defined by a1=2a_1 = 2 and an+1=3an+4a_{n+1} = 3a_n + 4. We want to find the general term ana_n of the sequence.

2. Solution Steps

We have the recursive relation an+1=3an+4a_{n+1} = 3a_n + 4. We can rewrite this as an+1+c=3(an+c)a_{n+1} + c = 3(a_n + c) for some constant cc. Expanding the right side, we have an+1+c=3an+3ca_{n+1} + c = 3a_n + 3c. Comparing this with an+1=3an+4a_{n+1} = 3a_n + 4, we have 3c=c+43c = c + 4, so 2c=42c = 4 and c=2c = 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, which means bnb_n is a geometric sequence with common ratio

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

Since bn=an+2b_n = a_n + 2, we have an=bn2=43n12a_n = b_n - 2 = 4 \cdot 3^{n-1} - 2.
We can check the first few terms:
a1=43112=412=2a_1 = 4 \cdot 3^{1-1} - 2 = 4 \cdot 1 - 2 = 2.
a2=43212=432=10a_2 = 4 \cdot 3^{2-1} - 2 = 4 \cdot 3 - 2 = 10.
Using the recursive relation, a2=3a1+4=3(2)+4=10a_2 = 3a_1 + 4 = 3(2) + 4 = 10.
a3=43312=492=34a_3 = 4 \cdot 3^{3-1} - 2 = 4 \cdot 9 - 2 = 34.
a3=3a2+4=3(10)+4=34a_3 = 3a_2 + 4 = 3(10) + 4 = 34.

3. Final Answer

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

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