+0  
 
0
71
3
avatar+156 

 

Please help! I'm stuck on this one. Thank you!

mathelp  Aug 17, 2018
 #1
avatar+91019 
+3

y = -0.000488x^2  + 0.656x

 

We have the form  y  =  ax^2  + bx

 

The x coordinate of the vertex  is =  -b / [ 2 *a ] =      - 0.656  / [ 2 *  -0.000488 ] ≈  672.13

To find the  height above the river, put this value back into  the function and we  get

y  = -0.000488(672.13)^2  + 0.656(672.13) ≈ 220.5 ft

 

 

The section of the bridge above the arch will be the difference between  the intercepts of the function and the x axis

But  (0,0)  is one intercept...and using the fact that this parabola is symmetric bout its vertex point....the other  intercept occurs at   2 ( 672.13)  = 1344.26 ft ≈  1344.3 ft.

 

See the graph, here :  https://www.desmos.com/calculator/jd1toj6pak

 

[The line y = 220.5  represents  the bridge span  between the two  x intercepts  ]

 

 

cool cool cool

CPhill  Aug 17, 2018
 #2
avatar+156 
+2

Thank you so much for your help! smiley

mathelp  Aug 17, 2018
 #3
avatar+91019 
+2

You're welcome  !!!

 

 

 

cool cool cool

CPhill  Aug 17, 2018

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