The vertex form of a quadratic polynomial with real coefficients is
 |
where 
. The function
graph 
is a parabola with vertex 
and axis of symmetry 
. It opens upward when 
and downward when 
(OpenStax 2025).
Completing the square rewrites a quadratic polynomial 
in vertex form as
 |
so 
and 
. For example, 
, whose function
graph has vertex 
.
