Where is written:

\( \sqrt {4} = 2 \\ \sqrt {4} \neq-2\)

^{ ?}

There are two square roots of 4: 2 and -2.

The positive square root is called the "principal root".

The square root sign is defined to mean the principal root; thus, sqrt(4) = 2, not -2.

Where is written?

Thank you Geno, but is there somewhere to read?

?

Try: http://mathworld.wolfram.com/PrincipalSquareRoot.html

Thank you Alan!

Everything is clear now.

!