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vertex of parabola

Select the x coordinate of the vertex of the parabola defined by the function: f(x)=-5x^2+3x+2
 
A. -5/3
B. 2
C. 3/10
D. -3/5
 
I keep coming up with 0. I know I have missed a step, please help.

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Parviz F. | Mathematics professor at Community CollegesMathematics professor at Community Colle...
4.8 4.8 (4 lesson ratings) (4)
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 Without calculus, the  Horizontal Coordinate of a quadratic of  f(x) =aX^2 + bx + c  is equal to - b/ 2a
   Which in this case of
       f( X) =  - 5 X^2 + 3X + 2  
 
       equals :    - 3/[2(-5)] = -3/ -10 = 3/10 
Vivian L. | Microsoft Word/Excel/Outlook, essay composition, math; I LOVE TO TEACHMicrosoft Word/Excel/Outlook, essay comp...
3.0 3.0 (1 lesson ratings) (1)
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Hi Christine;
f(x)=-5x2+3x+2
The vertex is the point at which the change of y is zero.  Therefore, we can take the first derivative of the function and calculate this...
0=-10x+3
-3=-10x
3/10=x
ANSWER C
Varaia R. | I was put on this earth so you finally can "get" math.I was put on this earth so you finally c...
5.0 5.0 (29 lesson ratings) (29)
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If you have a graphing calculator you can visually see that the answer is C. 
There is a formula derived from the quadratic formula for the x coordinate of the vertex which is -b/(2a)  
For any parabola it will give you the x coordinate of the vertex. 
 
f(x)=-5x^2+3x+2
a=-5, b=3, c=2
 
-3/(2*-5) = 3/10
 
 
The answer is C. 3/10