SOLUTION: The line of symmetry for the quadratic equation y = ax2 + 8x - 3 is x = 4. What is the value of "a"?

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Question 1181264: The line of symmetry for the quadratic equation y = ax2 + 8x - 3 is x = 4. What is the value of "a"?

Found 2 solutions by josgarithmetic, ikleyn:
Answer by josgarithmetic(39617) About Me  (Show Source):
You can put this solution on YOUR website!
If complete-the-square for equation,
y=a%28x%2B4%29%5E2-%2816%2B3a%29%2Fa
and vertex point would be ( -4/a, -(16+3a)/a ).

Symmetry axis x=4 means the x value for vertex is -4%2Fa.

a=-1

Answer by ikleyn(52781) About Me  (Show Source):
You can put this solution on YOUR website!
.

For the general form quadratic function

     y = ax^2 + bx + c


the x-coordinate of the vertex and the symmetry line is  x = -b%2F%282a%29.

In your case,  x = -+8%2F%282%2Aa%29 = -4%2Fa.


From the other hand side, you are given that  x = 4;  hence


    -4%2Fa = 4.


From this equation, you find   a = -4%2F4 = -1.    ANSWER


ANSWER.  a = -1.

Solved, answered, carefully explained and completed.