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Not sure why you might think it is an option - it just works
Swipe to the search window, type in your math operation, click Search, and the operation is solved.
Or, click the microphone, and dictate your math operation. Solved instantly.
What doesn't work for you?
 
You don't even have to tap search, it does the calculation live.

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Works on my iPad Air 2 even with no calculator. Try switching your language to English to test.
 
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