Two questions that look the same: Question Answer What is $\sqrt9$? $3$ Which real numbers satisfy $x^2=9$? $3$ and $-3$ Many people find the first line strange. If $(-3)^2$ also gives $9$, why isn’t $\sqrt9$ “$\pm3$”? The answer is in chapter 1, in the old name of the symbol. Radix quadrati was the side of the square. A square of area $9$ has side $3$, and no side measures $-3$.