A toy rocket was launched from the ground. The function f(x) = -16x2 + 128x shows the height of the rocket f(x), in feet, from the ground at time x seconds. What is the axis of symmetry of the graph of f(x), and what does it represent?

Question

anonymous · Answer

so the 'x' coordinate of our vertex is 4...
so I'm going to say the rocket would take 4 seconds to get up to maximum height...and then it will fall back down taking 4 seconds
How do i know it will only take 4 seconds to get down? Well I would find out the x-intercepts of the parabola by factoring
-16x^2 + 128x = 0
factor our like terms
-16x(x - 8) = 0
so we have 2 equations

-16x = 0
and
(x - 8) = 0
well that means our 2 x intercepts are 0 and 8

So if it take 4 seconds to go up to the max height...we only have 4 seconds to fall back down

Hence the answer being B

anonymous · Answer

Woah, Where did you get that answer from?

misty1212 · Answer

what on earth?

misty1212 · Answer

first coordinate of the vertex is always (i repeat always ) $$-\frac{b}{2a}$$

misty1212 · Answer

do 
$$-\frac{128}{2\times (-16)}$$ and that is all

anonymous · Answer

@misty1212  What he did was, He went on google, Searched up my question. And some other guy that has already posted this question answered it.

misty1212 · Answer

i love google, but $-\frac{b}{2a}$ cannot be that hard to compute 
in any case what does "the answer is B" mean in this context?

anonymous · Answer

Like this http://widget.openstudy.com/study#/updates/5344393ae4b02bb88e656693 Then he just takes credit for the answer

anonymous · Answer

Thanks @LesTwins  I do the same. I already got that answer but I want to double check. Quite slick.

anonymous · Answer

I did the math then searched with google to give u the insurance you need to get the question correct