# Math Help - Cubic Function behaviour

1. ## Cubic Function behaviour

I say the answer is d) . But the book says its b) ?
Who is wright?

2. ## Re: Cubic Function behaviour

would it be so hard to rotate the picture before you post it? Really....

3. ## Re: Cubic Function behaviour

It wouldn't be so terribly difficult to type the whole thing so people do not have to try to read photographs!

Which statement is true for any cubic polynomial function?
(a) As x goes to +/- infinity, y goes to infinity.
(b) As x goes to +/- infinity, the signs are opposite.
(c) As x goes to +/- infinity, y goes to -infinity.
(d) As x goes to +/- infinity, the signs are the same.
Did you give this much thought at all? A simple cubic function is $f(x)= x^3$. "10000" isn't "infinity" but is pretty large! What is f(10000)? What is f(-10000)?

4. ## Re: Cubic Function behaviour

f(-10000) = -10000^3 , the answer is negative. So it is option d) ???

5. ## Re: Cubic Function behaviour

why don't you go through each of those choices and say whether or not it's true and if not why not. That will let you truly understand that problem.

6. ## Re: Cubic Function behaviour

When it says the signes are the same what does that mean?

7. ## Re: Cubic Function behaviour

Originally Posted by sakonpure6
When it says the signes are the same what does that mean?
that's a good question. It's terrible terminology. What I assume it means is that

$(x>0) \longrightarrow (x^3>0) and (x<0) \longrightarrow (x^3 < 0)$

8. ## Re: Cubic Function behaviour

I am really convinced its option d) because x -> infinity then y -> infinity and when x -> -infinity then so will y.

9. ## Re: Cubic Function behaviour

Originally Posted by sakonpure6
I am really convinced its option d) because x -> infinity then y -> infinity and when x -> -infinity then so will y.
well that's good because (d) is indeed correct.

10. ## Re: Cubic Function behaviour

thank you all!

11. ## Re: Cubic Function behaviour

Originally Posted by romsek
well that's good because (d) is indeed correct.
What? (d) is
(d) As x goes to +/- infinity, the signs are the same.
but sakonpure6 has already said that f(-10000) is negative while f(10000) is positive.

12. ## Re: Cubic Function behaviour

@Halls I am really confused, can you please explain your self?

13. ## Re: Cubic Function behaviour

You said earlier you did not know what "the signs are the same" meant. f(1000) is positive. Its "sign" is "+". f(-1000) is negative. Its sign is "-". The signs are NOT "the same".

14. ## Re: Cubic Function behaviour

I think the trouble is in the terrible wording of the multiple choices.

If one assumes that "the signs are the same" means what I said a few posts back then (d) is correct. If not, well you'll have to define what it means then.

so you don't have to scroll back..

$(x>0) \longrightarrow (x^3>0) and (x<0) \longrightarrow (x^3 < 0)$