Solving the Equation y=-2x^3-9x^2-60x

[marlon]

Marlon you apparently have no idea what a function is. I see that you have read the formal definition, that is good. But, apparently it’s meaning was lost on you.

A function simply is some transformation on a thing (let’s call it a dependant variable) turning into another thing (let’s call it a dependant variable). The function also has another requirement, when you transform an independent variable you only get one result.

So when you ask me to solve

f(x)=-2x^3-9x^2-60x

With out telling me what you want me to transform that independent variable it into, you aren’t supplying sufficient information.

The “-2x^3-9x^2-60x” part is what the transformation this particular function is. It takes an input, cubes it then, multiplies it by –2. After that it takes that same input squares it and multiplies it by –9. Then it takes the input and multiplies it by –60. And lastly it takes those three values and adds them together. What you wanted to know is when will this process give a result of 0. Another equally valid question is when will this process give a result of 10? Or 20?

Functions are very different from equations. With an equation you are trying to find a solution. Functions are entirely different ideas. With functions you give an input and get an output.

For example the function of your height over time could be: for all 0<t<20: f(t) = t(t-20)^(1/2) where f(t) is in inches and t is in years. It makes no sense to solve this equation for 0. Why would you want to know when you were 0 inches tall? But you might want to find out when (or if) you were going to be 6 feet tall. Which would be 72= t(t-20)^(1/2)


Back to your function. What you wanted to ask is what independent variable will make the function yield a value of 0. I.e. solve f(x)=0

But let’s say you wanted to figure out when the function gave a value of, oh 20. I.e. f(x)=20

Then you would get: 20=-2x^3-9x^2-60x
0=-2x^3-9x^2-60x-20

really?



ps : make sure that if you want to correct someone, you do it the right way. Your definition of a function is not complete. For example can x = 6 be catagorized as a real function conform is mathematical definition? Besides a function is a relation and not a transformation.

Obviously you need to be more correct in your corrections

marlon