Example

Finding the Equation of a Parabolic Arch Bridge

To find the equation of a parabolic arch, such as in a bridge foundation, first establish a coordinate system. For a bridge that is 1010 feet high and 2020 feet wide at its base, placing the lower left side at the origin (0,0)(0, 0) makes the lower right side (20,0)(20, 0). The vertex, which is the highest point, falls halfway between the base points due to symmetry, making its xx-coordinate 1010. Since the height is 1010 feet, the vertex is (10,10)(10, 10). Substitute the vertex (h,k)=(10,10)(h, k) = (10, 10) into the standard form equation y=a(xβˆ’h)2+ky = a(x - h)^2 + k to get y=a(xβˆ’10)2+10y = a(x - 10)^2 + 10. Next, substitute another point on the parabola, such as (0,0)(0, 0), into the equation to solve for aa: 0=a(0βˆ’10)2+100 = a(0 - 10)^2 + 10. Solving this yields βˆ’10=100a-10 = 100a, so a=βˆ’110a = -\frac{1}{10}. Substituting aa back into the equation gives the final standard form: y=βˆ’110(xβˆ’10)2+10y = -\frac{1}{10}(x - 10)^2 + 10.

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Updated 2026-05-26

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