The problem asks us to find the general term $a_n$ of a sequence defined by the recurrence relation $a_{n+1} = 3a_n + 4$ with the initial condition $a_1 = 2$.

AlgebraRecurrence RelationsSequencesGeometric SequencesAlgebraic Manipulation
2025/3/17

1. Problem Description

The problem asks us to find the general term ana_n of a sequence defined by the recurrence relation an+1=3an+4a_{n+1} = 3a_n + 4 with the initial condition a1=2a_1 = 2.

2. Solution Steps

First, let's rewrite the recurrence relation in the form an+1+α=3(an+α)a_{n+1} + \alpha = 3(a_n + \alpha). Expanding the right side, we get an+1+α=3an+3αa_{n+1} + \alpha = 3a_n + 3\alpha. Comparing this to an+1=3an+4a_{n+1} = 3a_n + 4, we have α=3α4\alpha = 3\alpha - 4, which means 2α=42\alpha = 4, so α=2\alpha = 2.
Therefore, the recurrence relation can be written as an+1+2=3(an+2)a_{n+1} + 2 = 3(a_n + 2). Let bn=an+2b_n = a_n + 2. Then the recurrence relation becomes bn+1=3bnb_{n+1} = 3b_n. This is a geometric sequence with common ratio 33. We have b1=a1+2=2+2=4b_1 = a_1 + 2 = 2 + 2 = 4. The general term for a geometric sequence is given by
bn=b1rn1b_n = b_1 r^{n-1},
where b1b_1 is the first term and rr is the common ratio. In this case, b1=4b_1 = 4 and r=3r = 3, so
bn=43n1b_n = 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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