Inline and display equation font size

I need the display equation fontsize 1pt smaller than the normal text, but inline equation remain same as text font size. how to handle this in latex anyone help on this

You can define a new environment.

\documentclass[11pt]{article}
\usepackage{amsmath}
\usepackage{environ}
\NewEnviron{myequation}{%
\par\vspace{-\baselineskip}
\small
$$% \BODY$$
}

\begin{document}
\noindent Some text here $y=x$ Some text here Some text here Some text here Some text here Some text here Some text here Some text here Some text here Some text here
\begin{myequation}%
f(y) = ya \times k^{y} %
\end{myequation}
Some text here $f(y)=0$
\end{document}


• this has the unfortunate side effect that, if the paragraph preceding the "smallified" equation has multiple lines, it will have the baselines corresponding to the small font, not the normal spacing. take a look at Reducing font size in equation. – barbara beeton Apr 2 '14 at 14:00
• @barbarabeeton Thanks for noting it. I made some ugly correction (which I am sure, isn't correct but something is better than nothing). I am not sure about the \vspace amount though. – user11232 Apr 2 '14 at 14:07

Define a newcommand for setting equation font size.

\documentclass[10pt]{article}
\newcommand{\se}[1]{\fontsize{9}{0} % 1pt smaller then normal text. Set it whatever you want
$${#1}$$
\fontsize{10}{0}
}

\begin{document}

This is $f(x)=g(x)$ an inline equation.

This is display mode equation. \se{\lim_{x \to \infty} \exp(-x) = 0}

This is another $y=\sin (x)$  inline equation.

\end{document}