Calculator limits on a parabola

Question

Hi guys I'm making Patrick Star for a graphing project.

Anyways I'm using a parabola for his head on my TI-84 but when I set limits on it, it graphs a straight line.

So the equation itself is $y=(-0.5(x+1)^2+9)$ and I set limits to {$2.7$ is greater than $x$} and {$-6.9$ is less than $x$}

At $x=2.7$ it makes a horizontal line to infinity and at $x=-6.9! it makes another horizantal line to negative infinity.

Does anyone know how i can stop this?

Answer 1

It's not clear exactly what you did. If you were doing something like

Y = (-0.5(X+1)² + 9)(2.7>X)(-6.9<X)

then this will cause the value of Y to be zero when X is outside the selected range (because one of the two operators > or < will produce the value $0$), but it will not make the value of Y be undefined, which is what you want.

You might try parametric graphing. For example:

X = T
Y = -0.5(T+1)² + 9

and for the WINDOW variables,

Tmin = -6.9
Tmax = 2.7

(Caveat: I'm inferring all of this from the TI-84 manual and from hints at https://answers.yahoo.com/question/index?qid=20080612194029AAThWAe. I don't have the actual calculator on hand to try it, so you may need to do a little experimenting. For example, will the calculator allow you to set Tmin to a negative number?)