The square root of 16 is 4. Why isn't it -4 ?
Actually, it is!
The form for a square root (otherwise known as a radical) is
r^2 = x
which means that the square root of x is a number r, which when multiplied by itself (r * r) yields x.
So in this case, 4^2 = (4 * 4) = 16, but also
(-4)^2 = (-4 * -4) = 16
So any positive real number, like 16, actually has 2 square roots, one positive, the other negative. The positive square root is termed the "principal square root," the negative square root is unfortunately unnamed. So there's a bit of a misnomer; when you say "square root" it really refers to both negative and positive roots, but it has come to mean only the positive square root.
If you write `sqrt(16)` this is equal to 4. This is because the `sqrt(n)` without a sign in front signifies the principal square root which is always positive. To signify both the positive and negative square roots we use `+-sqrt(16)` which is equal to both +4 and -4. But when you ask in words what is the square root of 16 it is 4 or -4. The reason for this is that `y=sqrt(x)` is a function, while `y=+-sqrt(x)` is not a function because one value of x gives two outputs.
Hope that helps...