Calculus: Derivatives Problem #2

[Hothot330]

[SOLVED] Calculus: Derivatives Problem #2

Homework Statement

The graph of y=2x^3+24x-18 is

Homework Equations

y '=6x^2+24
y ''=12x

The Attempt at a Solution

The answer to this is "increasing for all values of x"
But I want to know why...if substituting -1 for x will make the problem a negative.

 

[Dick]

What's the question? If you want to find where y is increasing it's where is y' positive. You don't think y' is positive for all x?

 

[Hothot330]

because when you plug in -1 for y=2x^3+24x-18 the end result is a negative not positive.

The other choices to the questions are:
b.decreasing for all values of x
c.only increasing for values of x on the interval (-infinity,-2)U(2,+infinity)
d.only increasing for values of x on the interval (-2,2)
e. only decrasing for values of x on the interval (-infinity,-2)

 

[Hothot330]

NVM, TOTAL IDIOT HERE... I know what you're saying now. Thanks.

 

[Dick]

The question isn't where y is negative, it's where y in increasing. y can be increasing even if it's negative.