Some mathematical elements change their style depending on the context, whether they are in line with the text or in an equation-type environment. This article explains how to manually adjust the display style.
Let's see an example
Depending on the value of $x$ the equation \( f(x) = \sum_{i=0}^{n} \frac{a_i}{1+x} \) may diverge or converge.
\[ f(x) = \sum_{i=0}^{n} \frac{a_i}{1+x} \]
Superscripts, subscripts and fractions are formatted differently.
The maths styles can be set explicitly. For instance, if you want an in-line mathematical element to display as a equation-like element put \displaystyle
before that element. There are some more maths style-related commands that change the size of the text.
In-line maths elements can be set with a different style: \(f(x) = \displaystyle \frac{1}{1+x}\). The same is true the other way around:
\begin{eqnarray*}
\begin{eqnarray*}
f(x) = \sum_{i=0}^{n} \frac{a_i}{1+x} \\
\textstyle f(x) = \textstyle \sum_{i=0}^{n} \frac{a_i}{1+x} \\
\scriptstyle f(x) = \scriptstyle \sum_{i=0}^{n} \frac{a_i}{1+x} \\
\scriptscriptstyle f(x) = \scriptscriptstyle \sum_{i=0}^{n} \frac{a_i}{1+x}
\end{eqnarray*}
\end{eqnarray*}
For more information see