Question

f(x) = 9x-4ln(x) X>0

I get the critical point of 4/9...

For some reason I'm screwing up finding the interval where x is increasing and decreasing. I'm tired! Please help.

Answer 1

When testing x values do I use f(x) or f'(x)?

Answer 2

f'(x) > 0 means f(x) is increasing

Answer 3

And then test to the right and left of 4/9, right?

Answer 4

i draw a number line and mark the zeros. then i check the intervals... sign check is sufficient since you only need + or -

Answer 5

Can you check if my deriv is wrong? I get f'(x )= 9-4/x...

Answer 6

I know the answer is that it is decreasing from (0,4/9) but I can't get it to be a negative number in my calculations lol

Answer 7

f'(x) = 9 - 4/x = 0 => x = 4/9

Answer 8

Okay! Whoops I see my error! Thanks a lot, I was testing with numbers lower than 0, whoops!

Answer 9

Thanks a lot!

Answer 10

you're welcome