Question

What is the axis of symmetry of the function f(x) = –(x + 9)(x – 21)?

Answer 1

First, we have to realize that this is factored form, so we have to put this in standard quadratic form using FOIL, which is a rule mandating how we multiply the numbers here. F stands for First, which means the first number inside the parentheses for both of them, then O means the first number in the first quantity(fancy word for parentheses) multiplied by the last number of the second quantity, and so forth. When you are done, you combine like terms

anyway, you end up with x²-12x-189, but you have to inverse all numbers because of the parentheses in front, so it becomes -x²+12x+189.

Now you can find the axis of symmetry uisng the equation -b=2a (oh yeah, I forgot to mention: the standard quadratic form is a(x)²+b(x)+c already). So you do -12 / -2, which then becomes 6.

The axis of symmetry is 6