# Function ?

1. Oct 28, 2004

### edieber

how do I solve this one?
y=-2x^3-9x^2-60x

2. Oct 28, 2004

### Muzza

You're going to have to supply more information about the problem. The actual question would be good.

3. Oct 28, 2004

### edieber

function inquiry question?

this it function inquiry question?

4. Oct 28, 2004

### omicron

I don't think that is gonna help.

5. Oct 28, 2004

### T@P

you want to solve for the roots of this function?

in that case, your function is
$$0 = -2x^3 -9x^2 -60x$$ solving for x
in that case, factor out x from the right hand side,
you get $$x \cdot (-2x^2 -9x -60) = 0$$
then you can use the quadratic formula to solve for x.
x would equalt to $$\frac{-b \pm \sqrt{b^2 -4ac}}{2a}$$
which by pluging and chuging gives you two answer.
The final answer would then be x = 0, $$r_{1}$$, $$r_{2}$$

in the case that you are attempting to simply draw it with the help of calculus, i do not see where the problem lies. Simply take the first derivative, set it to 0, and continue normally. perhaps a little more information on what your desired answer is...?

6. Oct 28, 2004

### marlon

Perhaps, i am missing something here, but what's all the fuss about???

Just write : -x(2x^2+9x+60) = 0. So 0 is one solution. Then you have to solve 2x^2+9x+60 = 0. Since D = 81-2*4*60 < 0 You have no real solutions. can you incorporated compelx numbers ? If so, just continue as in the D > 0 case. if not, there are no solutions here...

marlon

7. Oct 28, 2004

### T@P

good pont marlon, there is only one solution. it is x = 0. sorry i didnt even bother to check what D was.

8. Oct 28, 2004

### Muzza

Most of us are not mindreaders, and don't know what people are really asking when they say "solve" and then post a function.

9. Oct 28, 2004

### marlon

Sorry, but do you know another way to solve a function ???

i think not...

10. Oct 28, 2004

### Muzza

I don't know of any way to "solve a function", just like I don't know how to "solve a banana".

11. Oct 28, 2004

### marlon

mmmmm

sweden, ja???

are you into ABBA ???

marlon

12. Oct 28, 2004

### Muzza

Nope. How many waffles have you eaten today...?

13. Oct 28, 2004

### marlon

5

marlon

14. Oct 29, 2004

### dav2008

y=x

solve that function

15. Oct 30, 2004

### marlon

marlon

16. Oct 30, 2004

### marlon

Solving means finding the solutions guys...

wow, what a revealing theory...

marlon

17. Oct 30, 2004

### dav2008

That answer is only true if y is 0.

18. Oct 30, 2004

### marlon

yes indeed it is...

marlon

19. Oct 30, 2004

### marlon

if y were to be 5 the the question should be : solve x - 5. hence the answer is 5...

beware that you do not violate the injectivity part of the definition of a function. With one x value, there can be at least one y-value, otherwise we do not have a function. Eg. x = 6 is NOT a function. y = x - 6 sure is...
Ofcourse with one y-value there can be several x-values like in y = x²

marlon

20. Oct 30, 2004

### HallsofIvy

Staff Emeritus
"Solving means finding the solutions guys...

wow, what a revealing theory...

marlon"

You still haven't understood what everyone is saying! Yes, "solve" means find the solutions- but to what problem?

The orginal post just said "How do I solve this one? y= 3x<sup>3</sup> -9x<sup>2</sup>- 60x"

That's not a "problem" that's just a statement. You then ASSUMED that the problem was "find all values of x that make y= 0" but that certainly is NOT the only possible problem that could be associated with a function.