he difficulty of graphing a quadratic function varies depending on the form you find it in. We'll start things off relatively easily.
f(x) = a(x – h)2 + k
No, we're not lying to you; that is a quadratic function. Go ahead, multiply it out.
f(x) = ax2 + (-2ah)x + (ah2 + k)
The h and k are constants, so (-2ah) and (ah2 + k) are also constants, which we could call, say, b and c. See, you can trust us, it's totally quadratic.
When you have a parabola written out like f(x) = a(x – h)2 + k, it's in vertex form. With vertex form, you have several pieces of important information thrown at you right away. When you first meet someone, your first impression tends to stick with you. It's the same with this equation.
The sign of a tells you if the parabola opens up or down. If a is positive, it opens up. If a is being a Negative Nancy, the parabola opens down. You also know that the vertex of the parabola is at the point (h, k). Be careful with the sign of h, though.