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2 Comments Already

mygif
minnytink Said,
February 5th, 2011 @8:09 pm  

You need to do your homework on your own. If you really don’t get it, ask your teacher, or go to a place like hotmath.com I think they can help you with your homework.

mygif
hayharbr Said,
February 5th, 2011 @8:34 pm  

You can see that (4,80) is the vertex (highest point). The formula for a parabola, which is the graph of a quadratic function, is

y = a(x-h)^2 + k where (h,k) is the vertex

So you would have y = a(x-4)^2 + 80

If you so this out by FOIL, it’s a(x-4)(x-4) + 80

y = a(x^2 – 8x + 16) + 80

Now you can plug in any other point to find a. Try (0,0)

0 = a(0 – 0 + 16) + 80

0 = 16a + 80

16a = -80 ; a = -80/16 = -5

So y = -5( x^2 – 8x + 16) + 80

y = -5 x^2 + 40x – 80 + 80

y = -5 x^2 + 40x

Replace y with h(t) and x with t and you have it

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