Question 1150069
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Place the origin of the coordinate system on the ground level under at the vertex of the parabola, 
i.e. under the top point of the arc.


Let x be horizontal axis and y be the vertical axis, directed up.


Then the equation of the parabola is  y = -ax^2 + 15.


Then the span of the parabola is from x = -90 m to x= 90 m.


Hence,  y = 0 = {{{-a*90^2^2 + 15}}},   or


    a*90^2 = 15,

    a = {{{15/90^2}}} = {{{15/(90*90)}}} = {{{1/(6*90)}}} = {{{1/540}}}.


Thus the equation of the parabola is  y = {{{-(1/590)*x^2 + 15}}}.   <U>ANSWER</U>
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Solved.