Question 446816
 {{{ C(x) = x^2 - 40x + 3200 }}}
You don't need to set {{{C(x) = 0}}}
That's only if you want the roots 
The minimum of the curve is at {{{ x = -b/(2a) }}}
when the form is {{{ ax^2 + 2b + c = 0 }}}
{{{ -b/(2a) = -(-40)/(2*1) }}}
{{{ -b/(2a) = 20 }}}
This is 20 fixtures daily
Now I find {{{C(20)}}}
{{{ C(20) = 20^2 - 40*20 + 3200 }}}
{{{ C(20) = 400 - 800 + 3200 }}}
{{{ C(20) = 2800 }}}
The minimum daily production cost is $2,800
Here's the plot:
{{{ graph( 500, 500, -10, 50, -500, 3600, x^2 - 40x + 3200) }}}