Byron S. answered • 10/09/14
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When you have a quadratic polynomial (ax2 + bx + c) that represents a parabola, you can find the x-coordinate of the vertex using the formula x=-b/(2a).
In your case, a=-9, and b=5. Can you figure out the x-value now?
Byron S.
The formula itself has a negative sign in it, so -5 ÷ [2(-9)] will make a positive number. Otherwise, you've got it right!
In bigger font:
x = -b / (2a)
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10/09/14
Roberto C.
Byron, with due respect, I don't think so, because -9x^2 + 5x + 7 can not be factored as a binomial squared. This problem requires to express the parable in the form a(x - h)^2 + k and the vertex will be (h,k).
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10/09/14
Byron S.
Roberto, the formula I gave is a shortcut to finding the vertex without completing the square. What I gave doesn't require factoring in any way, just plugging in the values. If you go through the process of completing the square on ax2 + bx + c, you'll find that h = -b / (2a).
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10/09/14
Byron S.
I should also note that it only finds the x-coordinate of the vertex. To find the y-coordinate (k), you must substitute the value you find back into the original function. This problem doesn't ask for that, however, so I didn't feel the need to elaborate on it.
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10/09/14
Jason S.
Great answer Byron. Simplest is best. I only added my answer as an example of why your answer is the best. :)
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10/10/14
Kristen P.
10/09/14