The simplest way to get a slightly more acceptable typeset result is to use TraditionalForm
as follows, after you enter the expression as in the question:
\!\(
\*UnderoverscriptBox[\(\[Sum]\), \(i = 1\), \(n\)]
\*SubscriptBox[\(x\), \(i\)]\)/\!\(
\*UnderoverscriptBox[\(\[Sum]\), \(i = 1\), \(n\)]
\*SubscriptBox[\(y\), \(i\)]\) == \!\(
\*UnderoverscriptBox[\(\[Sum]\), \(i = 1\), \(n\)]\((\((
\*FractionBox[
SubscriptBox[\(x\), \(i\)],
SubscriptBox[\(y\), \(i\)]])\) p)\)\)/n
Magnify[%] // TraditionalForm

The reason this doesn't look exactly like what you want is that it wasn't wrapped in HoldForm
. For more discussion of how to conveniently enter math expressions especially in the context of graphics labeling, see this answer.
Here is a better way to enter things, but I better show just the image:


Edit 2
In the other answer I linked above, I mentioned another way to input formulas that actually belongs into the context of this more general question as well. Therefore, I've made a screen capture to illustrate the steps for getting what I think is the closest to a typesetting interface with Mathematica.
The screen movie has to be short because it's a GIF
animation:

- I'm starting with the expression typeset in an input cell.
- In the next cell I add
TraditionalForm
content with a dummy string
- The content of the string (
aaa
inside the quotation marks) is converted to TraditionalForm
via the menu
- The original expression is pasted into the invisible
FormBox
that was now created around the aaa
- The resulting string can be edited further (e.g., I replace the
==
by =
, but you can do arbitrary edits here)
- With an optional
Magnify
appended, I evaluate the cell and get the desired result
TraditionalForm
, but that's probably not for everyone... You might want to try getting rid ofCourier
though. That's one ugly font. $\endgroup$Ctrl /
(unless you copy the cell expression) pasting this into mma converts it to $a/b$ when you actually want $\displaystyle\frac{a}{b}$ (there was an old question by Sjoerd on SO about something related to this) $\endgroup$Courier
is the default font (at least on Mac.) Changing it to a font with serifs helped, but had the kind of issues you suggest it might. $\endgroup$