Question

Help with a Cal. Problem.

Answer 1

f'(x) = 2 - 2.216.x^(-3) > 0
means that
x^3 > 216 when x^3 > 0, i.e., when x > 0
and
x^3 < 216 when x^3 < 0 ....

Hence ...

Answer 2

No, be very careful here. You have to make sure that when you multiply or divide by x^3 that you look at the sign.

Answer 3

so x is negative or x > 6

Answer 4

@david: cool picture btw

Answer 5

Thanks!
Hmm, so it would be C, right?

Answer 6

No, not c

Answer 7

b

Answer 8

Okay, I see it now. x<0 & x<6....
Alright, Thanks guys!

Answer 9

No ... look at my two cases above:

x^3 > 216 when x^3 > 0, i.e., when x > 0
and
x^3 < 216 when x^3 < 0 ....

Answer 10

x^3 > 216 when x^3 > 0, i.e., when x > 0....

Answer 11

In first case, that means x > 6 and x > 0, i.e., x > 6

The second case is x < 6 and x < 0, i.e., x < 0

Answer 12

so the answer is b?

Answer 13

Yes, b.

Answer 14

yay

Answer 15

@agdgd: to chat

Answer 16

So to go about the problem, I would need the set the derivative > 0, and then solve for x, right?

Answer 17

Yes, which is what I was doing above. But you should definitely feel comfortable with all the calculations and logic yourself

Answer 18

Okay, Thanks again!