A family of Microsoft word processing software products for creating web, email, and print documents.
First, you need to add argmin to the list of recognized functions so the equation editor won't make it italic. Click the little arrow in the bottom right corner of the Conversions group on the Equation ribbon to open the Equation Options dialog, and click the Recognized Functions button in that dialog to open the Recognized Math Functions dialog. Type argmin in the empty box and click the Add button, then click OK in each dialog.
When you get to the right side of the equation, click the Limit and Log button on the ribbon and select the min function.
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When that function appears in the equation, move the cursor to its left and type arg. Because you've already told Word to recognize argmin as a function, it will be accepted. You can now use the placeholder below argmin to enter the β,a and use the other placeholder to enter the summation.