\displaystyle - Used to over-ride automatic style rules and force display style.
{ \displaystyle ... }
\displaystyle command is used to over-ride automatic style rules and force display style; stays in force until the end of math mode or the braced group, or until another style is selected.
\frac ab+\displaystyle\frac ab+\textstyle\frac ab +\scriptstyle\frac ab+\scriptscriptstyle\frac ab
\frac ab + {\displaystyle \frac cd + \frac ef} + \frac gh
\frac ab + \displaystyle{\frac cd + \frac ef} + \frac gh