Within Word 2007 equations, all characters use the same font by default. You
can set it through the 'Font' group on the 'Home' tab.
If you want to format individual characters, you have to type them as normal
text. On the 'Design' tab (visible when your pointer is inside the equation)
you can select 'Normal Text' from the 'Tools' group to insert characters
which you can format individually. You can use this functionality wherever
you want in your function.
Yves