# MATH / 

task_id: 1176352c-1064-5a1c-95a9-2659d887243f
task_key: precalculus--test--1176352c-1064-5a1c-95a9-2659d887243f
task_revision_id: 1

{"level":"Level 5","problem":"Let $M_n$ be the $n \\times n$ matrix with entries as follows: for $1 \\le i \\le n$, $m_{i,i} = 10$; for $1 \\le i \\le n - 1$, $m_{i+1,i} = m_{i,i+1} = 3$; all other entries in $M_n$ are zero. Let $D_n$ be the determinant of matrix $M_n$. Find\n\\[\\sum_{n=1}^{\\infty} \\frac{1}{8D_n+1}.\\]Note: The determinant of the $1 \\times 1$ matrix $[a]$ is $a$, and the determinant of the $2 \\times 2$ matrix $\\left[ {\\begin{array}{cc}\na & b \\\\\nc & d \\\\\n\\end{array} } \\right] = ad - bc$; for $n \\ge 2$, the determinant of an $n \\times n$ matrix with first row or first column $a_1$ $a_2$ $a_3$ $\\dots$ $a_n$ is equal to $a_1C_1 - a_2C_2 + a_3C_3 - \\dots + (-1)^{n+1}a_nC_n$, where $C_i$ is the determinant of the $(n - 1) \\times (n - 1)$ matrix formed by eliminating the row and column containing $a_i$.","type":"Precalculus"}

Source: https://github.com/hendrycks/math

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