Math Expression is possible to change the change the fonts

I'm interested in using Math Expressions to produce school books with a large number of formulas, but it just generates the math expression with one typeface only, and I really need them to be in different styles, Does anyone know how to change it?
There is any command to be insert in the MathML code.
I'm using a simple code here for fractions:

Thank you for your help.

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