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Consider the problem of finding the line of symmetry and vertex of the quadratic equation f(x) =x^2-8x+15 What is the error in the solution below? x^2-8x+15=0 x=-8/2 =-8/2=-4 line of symmetry 8^2-8x+15=0 (-4)^2-8(-4)+15=0 16+32+15=0 y=63 (-4,63) vertex A. The solution is correct. B. The line of symmetry should have been 4 instead of –4. C. The vertex is incorrect; it should have been {–4, 53}. D. -4^2 should have been squared as –16 instead of 16.

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We are given the quadratic:


f(x)=x^2-8x+15, with a=1, b=-8, c=15.


We know that the x-coordinate of the vertex, which is the point where the line of symmetry passes through is


\displaystyle{ -(b)/(2a).

Thus, the x-coordinate of the vertex is
-(b)/(2a) =-(-8)/(2\cdot1)= (8)/(2)=4.

Thus, the line of symmetry is x=4.


Answer: B. The line of symmetry should have been 4 instead of –4.
User Noah Gibbs
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