college algebra

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I have been stuck on this problem for over a week and I don't even know how to start the problem.

Parabola equation y= 4(100 - x^2/2500)
x and y are measured in feet

A. Find the length of the road across the bridge
B. find the height of the parabolic arch at the center of the span
C. Find the lengths of the vertical girders at intervals of 100 feet from the center of the bridge

( graph of upside down parabola)

problem is on photobucket
album: math problems

• college algebra -

so I will "guess" at a diagram
expanding and simplifying your equation I got
y = (-1/625)x^ + 400
b) at the centre, x = 0
so y = 400
the bridge is 400 ft high

width of bridge, let y = 0 (on the x-axis)
0 = (-1/625)x^2 + 400
x^2 = (625)(400)
x = ±25(20) = ±500
So the bridge is 1000 ft wide

for vertical girders
let x = 100, 200, 300 , 400

x = 100
y = (-1/625)(100)^2 + 400 = 384
x = 200
y = (-1/625)(200)^2 + 400 = 336
etc.

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