I need help

i got this function f(x)= -2x^3+8x, so i need to find the vertex but right now i'm confuse. using the -b/2a i got 2 and then i replacing and it was 0 so it's (2,0)?

Question

I need help 

i got this function f(x)= -2x^3+8x, so i need to find the vertex but right now i'm confuse. using the -b/2a i got 2 and then i replacing and it was 0 so it's (2,0)?

princeharryyy · Answer

$$f(x) = 2x(4 - x^2)$$

princeharryyy · Answer

$$y = 2x(2^2-x^2) = 2x(2-x)(2+x)$$

tkhunny · Answer

Notice how it is NOT a second-degree function.  It is Degree 3!!  -b/(2a) is not meaningful.

So, 1) Did you write it correctly?  Is it supposed to be -2x^2 + 8x?
2) Factoring might give greater insight.  Prince Harry has demonstrated that.
3) Do we get to use the calculus?

princeharryyy · Answer

https://www.google.co.in/webhp?sourceid=chrome-instant&rlz=1C1HLDY_enIN713IN713&ion=1&espv=2&ie=UTF-8#q=plot%20y%20%3D%202x(2-x)(2%2Bx)

princeharryyy · Answer

can you find vertex now.

mathmale · Answer

What course are you in?   If you have shared the correct function, f(x) = -2x^3 + 8x, you have a function involving the "cube of x," and cannot use the quadratic formula.  If you're studying calculus, you need only find the first derivative of this function, set it = to 0, solve for x, find the associated y values, and then you'll have the coordintes of the vertices (which happen to be your maximum and minimum).

If you're not yet in calculus, try estimating the coordinates of your two vertices.

mathmale · Answer

Note that    f(x)= -2x^3+8x  can be written in factored form    as prince-harry has already done.  If you'd set this factored form = to 0 and solve for the (3) values of x that satisfy this equation, you'll have the "x-intercepts."  that would be helpful in graphing this function.