# Definition:Binomial (Euclidean)/Second Binomial/Example

## Example

Let $a$ and $b$ be two (strictly) positive real numbers such that $a + b$ is a binomial.

By definition, $a + b$ is a second binomial if and only if:

$(1): \quad b \in \Q$
$(2): \quad \dfrac {\sqrt {a^2 - b^2}} a \in \Q$

where $\Q$ denotes the set of rational numbers.

Let $a = 2 \sqrt 3$ and $b = 3$.

Then:

 $\ds \frac {\sqrt {a^2 - b^2} } a$ $=$ $\ds \frac {\sqrt {12 - 9} } {2 \sqrt 3}$ $\ds$ $=$ $\ds \frac {\sqrt 3} {2 \sqrt 3}$ $\ds$ $=$ $\ds \frac 1 2$ $\ds \in \Q$

Therefore $2 \sqrt 3 + 3$ is a second binomial.