Quadratic Equation

I dont understand the formula -b/2a. I know quadratic equation ax2+bx+c but i dont know where this formula comes from?
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d-glitchConnect With a Mentor Commented:
a = -11     b = 100     -b/2a = (-100)/(2*(-11) = 50/11

f(50/11)  means evaluate the quadratic function at x = 50/11 to find the maximum.

Your calculation looks correct.  Your graph is not correct.

For example      f(x=0) is 8     f(x=1) is 87    f(x=-1) is -103  and the graph should be symmetric around x=50/11.
From calculus   x = -b/2a  is the value where the parabola will have its max or min value.  In this case (with a= -11) it will be a maxim.
Your graph is a sinusoid not a parabola.
If you use the quadratic formula you will find the (usually) two places where the function equals zero.  
x = -b/2a is halfway between the zeroes, and will be the location of the max (or min).
mustish1Author Commented:
what is this formula: f(50/11)
mustish1Author Commented:
Thank You.
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