How to insert mathematical eq.in textbox asp.net?

Text boxes are designed to carry only the plain text, without any formatting; so it highly limits the repertoire of expression you can write to something like

x³ = ln(y) + (x+1)√y

In this expression, I used no HTML markup, to mimic this situation, used only Unicode characters. (Sorry, even the root looks ugly.)

No, this is not a way to represent mathematical expression. You can do a lot more (but still, only pretty simple things) if you use the attribute contenteditable.
Please see the demo: http://html5demos.com/contenteditable[^].

Look at the HTML source of this page to see how to write such HTML.

And, finally, you can combine this idea with new HTML5 feature: the ability to natively show MathML. Please see:
http://en.wikipedia.org/wiki/HTML5[^],
http://en.wikipedia.org/wiki/HTML5[^].

MathML will allow to show really complex mathematical notation.

—SA