SOLUTION: A square centered at the origin has its vertices on the x and y axes. The graph of the function f(x) = ax2 - 4, a > 0 passes through three of the square's vertices. Find a.

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Question 571736: A square centered at the origin has its vertices on the x and y axes. The graph of the function
f(x) = ax2 - 4, a > 0
passes through three of the square's vertices. Find a.

I tried to draw a graph of it, but didn't really help, since I don't know the exact measures of the square... But being that its vertices are on the x and y axes I figured that it is like, diagonal, but other than that, I am completely lost!
Please help, I would definitely appreciate it :)

Answer by solver91311(24713)   (Show Source): You can put this solution on YOUR website!


Let the four vertices of the square (starting at the negative axis and going clockwise) be , , , .

Since the second degree term variable in the function is we know that the axis of symmetry of the parabola graph is a vertical line. Furthermore, since we are given that , we know that the parabola opens upward, and the vertex is a minimum local extrema. That fixes the three points of the square that are in common with the graph of the function as , , and with as the vertex of the parabola.

The standard form of a parabola is:



where

So if the parabola passes through the point then



In the case of your function at point ,



from which we deduce



But since the given figure was a square, we know that



Subsituting the coordinates of which we now know to be into your given function:



from which we get



John

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