To document mathematical formulas on a worksheet, you can insert or edit an equation as an object.
You build the equation by selecting symbols or structures from the Equation toolbar and by typing variables and numbers.
Insert an equation

On the Insert tab, in the Symbols group, click Equation.

Pick a common formula or the symbols and structures you want,
Edit an equation

Doubleclick the equation object that you want to edit.

Use the symbols, or structures to make your changes.