what is the answer to a^2=28

To solve this you need to find the inverse.

This inverse of squaring is square root.

So the square root of 28=a

$${\sqrt{{\mathtt{28}}}} = {\mathtt{5.291\: \!502\: \!622\: \!129\: \!181\: \!2}}$$

A= 5.29     ...Or that to be precise.

Note that there are two possible answers: a = ±√28

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