What is an example of an algebraic number of second order?

Question

I am looking for an example for an algebraic number of second-order.

I understand that we call any complex number is algebraic if it is a root for a polynomial.

Ex: $z^2+1=0$, then $z=i,-i$, so $i,-i$ are algebraic.

I want a brief explanation for an algebraic number of second order, and some examples.

Thanks in advance for any hint.

Answer 1

We say x is an algebraic number iff its a root of a monic polynomial with rational coefficients and the order of an algebraic number is the degree of its minimal polynomial. Look at the properties in Wikipedia page