Example

Finding the Equation of a Parabolic Arch (5 Feet High and 10 Feet Wide)

To write the equation of a parabolic arch that is 55 feet high and 1010 feet wide at its base, start by placing it on a coordinate system. If the lower left side of the arch is at the origin (0,0)(0, 0), the lower right side is at (10,0)(10, 0). By symmetry, the highest point of the arch occurs exactly halfway between the base points, giving an xx-coordinate of 55. Given the height is 55 feet, the vertex is (5,5)(5, 5). Substitute this vertex (h,k)=(5,5)(h, k) = (5, 5) into the standard form of a parabola, y=a(xβˆ’h)2+ky = a(x - h)^2 + k, to obtain y=a(xβˆ’5)2+5y = a(x - 5)^2 + 5. To determine the value of aa, substitute the coordinates of another known point on the arch, such as (0,0)(0, 0), into the equation. This results in 0=a(0βˆ’5)2+50 = a(0 - 5)^2 + 5, which simplifies to βˆ’5=25a-5 = 25a, giving a=βˆ’15a = -\frac{1}{5}. Substituting this value back in yields the final standard form equation: y=βˆ’15(xβˆ’5)2+5y = -\frac{1}{5}(x - 5)^2 + 5.

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Updated 2026-05-26

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