Thread: Sketching the graph

The following are procedures to draw a graph.

f'(x)>0 if |x|<2, f'(x)<0 if |x|>2, f'(2)=0, limit as x approaches infinity=1, f(-x)=-f(x), f"(x)<0 if 0<x<3, f"(x)>0 if x>3.

if the nature of the this function following |x|, what confuses me is how can i draw a graph that is concave downward between 0 and 3, and concave upward greater than 3 when the lines have to be straight.

-I did try drawing it but i am not sure if it's right. I begin at the origin and draw a line concave downward up to 2, from here should the turning point be pointy like on an |x| graph or smooth like on a parabola? then it continues to be concave downward with a negative gradient until it gets to 3 then it becomes concave upward.

-Also, for the limit as x approaches infiinity=1, does that mean there is a horizontal asymptote at y=1.

Hey johnsy123.

An example of a sudden change from concave upward to downward is that of a circle (although it technically isn't a function), but if you look at when y = 0, this is an example of something going from concave up to concave down.

An example of a function doing this would be if you the two semi-circles (one below the y axis and one above) and you shifted the one below the y-axis so that the left-hand point on the bottom semi-circle matched up with the right-hand side point of the top-circle (kind of like a skate ramp kinda thing): this is an example of a function going from concave down to concave up.

As for the horizontal asymptote, I think this is the case but the asymptote is going to be bounded from above and not below since the second derivative is always positive from x > 3 onwards.

I am aware of the nature of concavity. What i am buzzed on is should i sketch this graph like a |x| graph where the lines are linear? but if i do that then wouldn't it be impossible to draw concave downward and upward?

does the "f'(x)>0 if |x|<2" mean that because the |x| is there i have to draw this graph like that of a |x|?

here's what i tried anyway

Originally Posted by johnsy123

The following are procedures to draw a graph.

f'(x)>0 if |x|<2, f'(x)<0 if |x|>2, f'(2)=0, limit as x approaches infinity=1, f(-x)=-f(x), f"(x)<0 if 0<x<3, f"(x)>0 if x>3.

if the nature of the this function following |x|, what confuses me is how can i draw a graph that is concave downward between 0 and 3, and concave upward greater than 3 when the lines have to be straight.

-I did try drawing it but i am not sure if it's right. I begin at the origin and draw a line concave downward up to 2, from here should the turning point be pointy like on an |x| graph or smooth like on a parabola? then it continues to be concave downward with a negative gradient until it gets to 3 then it becomes concave upward.

-Also, for the limit as x approaches infiinity=1, does that mean there is a horizontal asymptote at y=1.

What it's saying is, that your function has the property for all , in other words, it has the property when .

It also has the property that if , in other wrods, that it has the property when and when .

Clearly that means when , in other words, has stationary points where .


Another important property is that as , so has a horizontal asymptote at .


It's an odd function, because , that means there is a reflection in BOTH the x and y axes.


Finally you're told that if and if . Recall that if the second derivative is negative at a stationary point, the stationary point is a local maximum. So that means at the point where you have a local maximum.


From the property of being an odd function, what does that tell you about the stationary point at ?

The stationary point at x=-2 is just the same as at x=2?