Discussion about math, puzzles, games and fun.   Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ • π ƒ -¹ ² ³ °

You are not logged in.

## #1 2006-01-03 11:05:42

Jacobpm
Guest

### Behavior of a graph at a point

Which of the following describes the behavior of y = cubicroot(x + 2) at x = -2

(A) differentiable
(B) corner
(C) cusp
(D) vertical tangent
(E) discontinuity

well i graphed the function, and i'm not sure.. i know for sure it isn't E... because f(-2) = 0... it has a value..  uhmm.. as for the rest i'm not sure.. i don't even know what a corner and a cusp is.

## #2 2006-01-03 11:15:29

krassi_holmz
Real Member

Offline

### Re: Behavior of a graph at a point

I don't know what is corner, but I think cusp is something like this:
I think (D)

IPBLE:  Increasing Performance By Lowering Expectations.

## #3 2006-01-03 11:22:27

Jacobpm
Guest

### Re: Behavior of a graph at a point

like what? your picture didn't show up

## #4 2006-01-03 11:22:46

krassi_holmz
Real Member

Offline

### Re: Behavior of a graph at a point

Plot:

Last edited by krassi_holmz (2006-01-03 11:23:24)

IPBLE:  Increasing Performance By Lowering Expectations.

## #5 2006-01-03 11:23:50

krassi_holmz
Real Member

Offline

### Re: Behavior of a graph at a point

Now it's better.

IPBLE:  Increasing Performance By Lowering Expectations.

## #6 2006-01-03 11:28:15

krassi_holmz
Real Member

Offline

### Re: Behavior of a graph at a point

For uploading an image direct to the forum use "post reply" or when you've written your quick post, simply edit it. There you can specify the number of images and the path to be uploaded.

IPBLE:  Increasing Performance By Lowering Expectations.

## #7 2006-01-03 11:32:47

Jacobpm
Guest

### Re: Behavior of a graph at a point

ok, so my grpah doesn't look like that.. so it doesn't have a cusp, correct?

so, hmm... we aren't sure about A, we aren't sure about B, we know it can't be C, we know it is D, and we know it can't be E..

So we're left with Discussions about A and B..

anything to add on those two?

## #8 2006-01-03 11:37:56

Jacobpm
Guest

### Re: Behavior of a graph at a point

I just noticed that it isn't differentiable either...

So the only thing i'm unsure of is about the corner

## #9 2006-01-03 12:35:36

krassi_holmz
Real Member

Offline

### Re: Behavior of a graph at a point

Yes, it isn't differenciable. And I don't know what is corner, too. Try at
http://www.mathworld.wolfram.com

Last edited by krassi_holmz (2006-01-03 12:36:10)

IPBLE:  Increasing Performance By Lowering Expectations.

## #10 2006-01-03 14:33:43

Ricky
Moderator

Offline

### Re: Behavior of a graph at a point

f(x) = abs(x), a corner exists at f(0).  It is literally a corner.

As for the answer, it is D:

For a verticle tangent, the function's slope must approach infinity as it approaches the point.

So what we want is:

Multiplying this by
we get:

Now x+2 approaches 0 as x approaches -2.  But we know that f(x)^(1/3) > f(x) if f(x) < 1.  So the numerator gets (relatively) larger and the denominator gets smaller as x approaches -2.  Therefore the slope approaches infinity, and you have a verticle tangent.

Last edited by Ricky (2006-01-03 14:36:15)

"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

## #11 2006-01-03 20:49:22

krassi_holmz
Real Member

Offline

### Re: Behavior of a graph at a point

D!

IPBLE:  Increasing Performance By Lowering Expectations.

## #12 2006-01-04 04:21:43

Ricky
Moderator

Offline

### Re: Behavior of a graph at a point

Oy, I completely missed a much more direct way to do the limit:

Since x+2 approaches zero as x approaches -2, x+2 is very close to 0 before it gets there, in other words, very small.  (x+2)^(1/3) then also becomes increasingly small, and multiplying this by 3 has basically no effect as it will also become increasingly small.  So 1 over this means it goes towards infinity.

Last edited by Ricky (2006-01-04 04:22:19)

"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

## #13 2006-01-04 21:51:57

krassi_holmz
Real Member

Offline

### Re: Behavior of a graph at a point

It depends of which side you are limiting. If x < -2 and x --> -2 then x+2<0 and 1/(x+2) --> -oo.
If x>-2 and x --> -2 then x+2>0 and 1/(x+2) --> +oo.

IPBLE:  Increasing Performance By Lowering Expectations.

## #14 2006-01-05 01:51:21

Ricky
Moderator

Offline

### Re: Behavior of a graph at a point

True, but in this case, it doesn't matter.  If it approaches negative infinity or infinity from either direction, it is still a vertical tangent.

"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

## #15 2006-01-05 02:05:11

krassi_holmz
Real Member

Offline

### Re: Behavior of a graph at a point

Yes, yes, just for exactude.

IPBLE:  Increasing Performance By Lowering Expectations.