How do you graph f(x)=(x^3-2x^2-3x)/(4x^2+8x) using holes, vertical and horizontal asymptotes, x and y intercepts?

1 Answer
Mar 20, 2018

Factor then analyze!

Explanation:

We can factor both the top and the bottom to get asymptotes, some intercepts, and holes.

x^3 - 2x^2 - 3x = x(x^2 - 2x-3) = x(x-3)(x+1)
4x^2 + 8x = 4x(x+2)

We see that they both have a zero in common (x=0), which means that x=0 is the one hole. If we consider the equation without the x term, we see that the function WOULD go through (0, -3 / 8), which is where the hole (and technically the y-intercept) is.

Since the bottom also goes to zero at x=-2, that's a vertical asymptote. Since the top goes to zero at x = 3, -1 those are zeros.

Last thing we need to observe: the top and bottom are different orders. Since the top is a higher order, in the limit, the equation will look like y=x/4.

Now we have everything except exact signs.
At xrightarrow -infty, the value is clearly negative so
y < 0 forall x < -2.

It then goes toward negative infinity and jumps to positive infinity after -2, until the next zero, i.e.
y > 0 forall -2 < x < -1.

At x=-1, it hits zero and switches sign, i.e.
y < 0 forall -1 < x < 0 or 0 < x < 3.

After x=3, the sign doesn't switch and the function quickly approaches that line, i.e.
y > 0 forall x>3 and y rightarrow x/4 .

From all of that analysis, you should be able to sketch a plot similar to this: graph{x(x-3)(x+1)/(4x(x+2)) [-13.86, 13.86, -6.93, 6.93]}