# How can I draw this shape: [duplicate]

I want to draw this matrix but I can't.

\documentclass{article}
$% \begin{array}{ccccc} a_1 & a_2 & a_3 & \ldots & a_n\\ 1 & 0 & 0 & \ldots & 0 \\ 0 & 1 & 0 & \ldots & 0 \\ 0 & 0 & 1 & \ldots & 0 \\ \vdots & \vdots & \vdots & \ddots & \vdots \\ 0 & 0 & 0 & \ldots & 1 \end{array}%$