tab is not working in latex

I am new in the world of latex . At this time , now I am writing thesis . I want to have tabs in tex document . For this purpose , I have written the following code .

\hspace{2cm}$\mathbf{a_{11}x_1 + a_{12}x_2 +. . . + a_{1n}x_n = b_1 }$ \newline
\hspace{2cm}$\mathbf{a_{21}x_1 + a_{22}x_2 +. . . + a_{2n}x_n = b_2 }$\newline
\hspace{2cm} ...     ...     ...   ...    ...  ... \newline
\medskip ...     ...     ...   ...    ...  ...\newline
\medskip ...     ...     ...   ...    ...  ...\newline
\medskip $\mathbf{a_{n1}x_1 + a_{n2}x_2 +. . . + a_{nn}x_n = b_n }$ \newline

But this code generates tab in first line and then in the next line the tabs is not generated . Why ? Please help .

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Welcome to TeX.SE. While code snippets are useful in explanations, it is always best to compose a fully compilable MWE that illustrates the problem including the \documentclass and the appropriate packages so that those trying to help don't have to recreate it. –  Peter Grill Jun 18 at 3:20

You can use \hspace*{} to get the desired effect:

However you really should use some sort of math environment such as align:

Code:

\documentclass{article}

\usepackage{amsmath}

\begin{document}
\noindent
\hspace{2cm}$\mathbf{a_{11}x_1 + a_{12}x_2 +. . . + a_{1n}x_n = b_1 }$ \newline
\hspace*{2cm}$\mathbf{a_{21}x_1 + a_{22}x_2 +. . . + a_{2n}x_n = b_2 }$\newline
\hspace*{2cm} ...     ...     ...   ...    ...  ... \newline
\medskip\hspace*{2cm} ...     ...     ...   ...    ...  ...\newline
\medskip\hspace*{2cm} ...     ...     ...   ...    ...  ...\newline
\medskip\hspace*{2cm} $\mathbf{a_{n1}x_1 + a_{n2}x_2 +. . . + a_{nn}x_n = b_n }$ \newline

\begin{align*}
a_{11}x_1 + a_{12}x_2 + \dotsb + a_{1n}x_n &= b_1 \\
a_{21}x_1 + a_{22}x_2 + \dotsb + a_{2n}x_n &= b_2  \\
\end{align*}
\end{document}
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Another solution using array and flalign allows for a better-looking long series of dots with the \hdotsfor command. In this context, a plain \dots will do the same job as \dotsb:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{fourier}
\usepackage{array}
\usepackage{mathtools, nccmath}

\begin{document}
\noindent Another solution: \useshortskip

\begin{flalign*}
\hspace{2cm} &
\begin{array}{@{}r@{} >{{}}l} \\
a_{11}x_1 + a_{12}x_2 + \dots + a_{1n}x_n &{} = b_1\\
a_{21}x_1 + a_{22}x_2 + \dots + a_{2n}x_n &= b_2 \\
\hdotsfor{2}
\end{array} &
\end{flalign*}

Text text text text text text text text text text text text text text text text text text text text text text text text text text text.
\end{document}

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