[Tex/LaTex] using less than or equal to symbol

equations

So this is my first LaTeX document. I am trying to write an in equality in this document. Given below is the code I have written

\documentclass[20pt]{article}

\usepackage{extsizes}

\begin{document}

Let f(x) be a real valued function of real variable such that  $ |f^n(0)|  \leq  K $

\end{document}

enter image description here
The code compiles without any error but where I should be getting a less than or equal to sign I instead get the letters leq. I have also tried using the code \le
. I also tried using the \usepackage{amsmath} command. But that generates an error undefined control sequence. Can somebody please correct this?

Best Answer

I hope this MWE will satisfy your request. Here there is a screenshot. You should also add the package for the symbols called amssymb to obtain \leq or \leqslant symbol.

enter image description here

    \documentclass[20pt]{article}
    \usepackage{amsmath,amssymb}
    \usepackage{extsizes}
    \begin{document}
    Let $f(x)$ be a real valued function of real variable such that

    \[ |f^n(0)| \leq K\]
    or \[ |f^n(0)| \leqslant K\] 

    \end{document}
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