回覆 3# kobelian 的帖子
第1題:
已知數列\(\langle\;a_n\rangle\;\)滿足\(a_1=3\),且對任意正整數\(n\)均有\(a_{n+1}=4+a_n+\sqrt{1+16a_n}\),則數列的一般式\(a_n=\)?
[解答]
\(\begin{array}{l}
{a_2} = 4 + {a_1} + \sqrt {1 + 16{a_1}} = 4 + 3 + 7 = 14\\
{a_3} = 4 + {a_2} + \sqrt {1 + 16{a_2}} = 4 + 14 + 15 = 33\\
{a_4} = 4 + {a_3} + \sqrt {1 + 16{a_3}} = 4 + 33 + 23 = 60\\
{a_5} = 4 + {a_4} + \sqrt {1 + 16{a_4}} = 4 + 60 + 31 = 95
\end{array}\)
注意到7,15,23,31成等差,因此可得
\[{a_n} = 4 + {a_{n-1}} + (8n-9) = a_{n-1} + 8n-5\]
則可得
\[\begin{array}{l}
{a_1} = 3\\
{a_2} = {a_1} + 11\\
{a_3} = {a_2} + 19\\
{\rm{ }} \vdots \\
{a_n} = {a_{n - 1}} + (8n - 5)
\end{array}\]
連加可得
\({a_n} = \displaystyle{\frac{{[3 + (8n - 5)] \times n}}{2}} = 4{n^2} - n\)