1. Given the sequence $\left\{a_{n}\right\}$ satisfies the first term $a_{1}=\frac{1}{2}, a_{n+1}$ $=3 a_{n}+7$. Find the general formula for the sequence $\left\{a_{n}\right\}$.
(Tip: Example 1. Answer: $a_{n}=4 \times 3^{n-1}-\frac{7}{2}$.)
