The equation x9 + 5x3 + 6 = 0 is not quadratic in form. A quadratic equation is typically expressed in the standard form of ax2 + bx + c = 0, where the highest exponent of the variable (x) is 2.
In this equation, the term with the highest exponent is x9, which clearly shows that it is of degree 9. This means the equation represents a polynomial of degree 9, as the highest exponent indicates the degree of the polynomial. Since the defining feature of a quadratic equation is the presence of a degree 2 term, this equation does not satisfy that criterion. Hence, it cannot be classified as a quadratic equation.