Example

Try It 9.125: Determining a Quadratic Function from its Graph

To find a quadratic function from its graph, identify the vertex and another point to substitute into the vertex form, f(x)=a(xh)2+kf(x) = a(x-h)^2 + k. For example, consider an upward-facing parabola with a vertex at (3,4)(3, -4) and a yy-intercept at (0,5)(0, 5). The vertex (h,k)(h, k) provides h=3h = 3 and k=4k = -4, yielding the partial equation f(x)=a(x3)24f(x) = a(x - 3)^2 - 4. To find the coefficient aa, substitute the coordinates of the yy-intercept (0,5)(0, 5) for xx and f(x)f(x): 5=a(03)245 = a(0 - 3)^2 - 4. Solving this equation gives 5=9a45 = 9a - 4, which simplifies to 9=9a9 = 9a, so a=1a = 1. Therefore, the specific quadratic function for this graph is f(x)=(x3)24f(x) = (x - 3)^2 - 4.

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Updated 2026-05-25

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