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Definition: A quadratic function is one of the form f(x) = ax^2 + bx + c, where a does not equal 0. Every quadratic function can be expressed in the form f(x) = a(x - h)^2 + k where (h,k) is the vertex of the function's graph.
Source: "Precalculus" by David H. Collingwood and K. David Prince at the University of Washington
Example: f(x) = x^2 f(x) = 5(x+3)^2 - 4