Consider the equation below. f(x) = x^7 lnx  Find the interval on which f is increasing. Find the interval on which f is decreasing.Consider the equation below. f(x) = x^7 ln x (Enter your answer...

Consider the equation below. f(x) = x^7 lnx 

Find the interval on which f is increasing.

Find the interval on which f is decreasing.

Consider the equation below. f(x) = x^7 ln x

(Enter your answer using interval notation.)

Asked on by agape801

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lfryerda | High School Teacher | (Level 2) Educator

Posted on

If you have more than one question, you need to make separate posts.

To find where a function is increasing and decreasing, you need to find where the derivative of the function is greater than zero or less than zero.

`f(x)=x^7lnx`  differentiate using product rule

`f'(x)=7x^6lnx+x^7/x`

`=x^6(7lnx+1)`

Since `x^6` is always positive, we need to determine when `7lnx+1` is greater than zero or less than zero.  We first find when it is equal to zero, then it will be positive on one side and negative on the other.

`7lnx+1=0`

`7lnx=-1`

`lnx=-1/7`  switch to exponential form

`x=e^{-1/7}`

This means that when `(e^{-1/7},infty)` , then the derivative is positive, which means the function is increasing, and when `(0,e^{-1/7})` , the derivative is negative, which means the function is decreasing.

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