The number [latex]\sqrt{7}[/latex] is an algebraic number, since it is the root of the equation : [latex]{x^2}– 7 = 0[/latex].
The complex (imaginary) number [latex]2[/latex]i = [latex]2\sqrt{-1}[/latex] is an algebraic number because it is the solution to the equation x2 + 4 = 0.