Quadratic polynomial
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In mathematics, a quadratic polynomial is a polynomial whose degree is (at most) 2. Some examples of quadratic polynomials are ax2 + bx + c, 2x2 − y2, and xy + xz + yz.
The coefficients of a polynomial are often taken to be real or complex numbers, but in fact, a polynomial may be defined over any ring.
When using the term "quadratic polynomial", authors sometimes mean "having degree exactly 2", and sometimes "having degree at most 2". If the degree is less than 2, this may be called a "degenerate case". Usually the context will establish which of the two is meant.
[edit] The one-variable case
If the polynomial is a polynomial in one variable, it determines a quadratic function in one variable. An example is given by f(x) = x2 + x − 2;. The graph of such a function is a parabola (in degenerate cases a line), and its zeroes can be found by solving the quadratic equation f(x) = 0.
[edit] The general case
In the general case, a quadratic polynomial in n variables x1, ..., xn can be written in the form
where Q is a symmetric n-dimensional matrix, P is an n-dimensional vector, and R a constant.
The zeroes of a quadratic polynomial form a quadric. The conic sections, such as ellipse and hyperbola, can be described with quadrics.
