Question 1038640
The points where the bridge meets the water are
( 15, 0 )
( -15, 0 )
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The form of the equation is:
{{{ y = -a*x^2 + b*x + c }}}
where {{{ a }}} is a positive number. This will make it a
parabola with a peak for a vertex, not a minimum.
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The x-value of the vertex is {{{ 0 }}} ( given )
{{{ x[v] = -b/(2a) }}} is the formula, so {{{ b = 0 }}}, since
{{{ x[v] = 0 }}}
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Find the y-value of vertex:
{{{ y[v] = -a*0 + 0 + c }}}
{{{ y[v] = c }}}
{{{ y[v] = 60 }}} ( given )
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So far,  I have:
{{{ y = -a*x^2 + 60 }}}
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When {{{ y = 0 }}}, {{{ x = 15 }}} or {{{ x = -15 }}}, so
I can say:
{{{ 0 = -a*15^2 + 60 }}} ( I could have used {{{ -15 }}} )
{{{ 225a = 60 }}}
{{{ a = 60/225 }}}
{{{ a = 12/45 }}}
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So the equation is: 
{{{ y = (-12/45)*x^2 + 60 }}}
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Here's the plot:
{{{ graph( 600, 400, -20, 20, -5, 70, (-12/45)*x^2 + 60 ) }}}
check:
When {{{ y=0 }}}, does {{{ x=15 }}} or {{{ x= -15 }}} ?
{{{ 0 = (-12/45)*x^2 + 60 }}}
{{{ (12/45)*x^2 = 60 }}}
{{{ x^2 = ( 45/12 )*60 }}}
{{{ x^2 = 2700/12 }}}
{{{ x^2 =  225 }}}
{{{ x = 15 }}}
{{{ x = -15 }}}
OK