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Find the Absolute Maximum and minimum values of f on the given interval.   f(x) = x -...

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carrawaych | Student, Undergraduate | (Level 1) Honors

Posted May 28, 2013 at 11:15 PM via web

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Find the Absolute Maximum and minimum values of f on the given interval.

 

f(x) = x - ln x  on [1/2,2]

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violy | High School Teacher | (Level 3) Assistant Educator

Posted May 28, 2013 at 11:52 PM (Answer #1)

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We will take the first derivative and equate the result to zero, to identify the critical point/s.

`f ' (x) = 1 - 1/x`

`f'(x) = (x - 1)/x` (Taking note that the lcd is x).

`x - 1 = 0`

Add 1 on both sides. 

`x = 1` So, we have a critical point at x = 1.

Take note that we will also have a critical point on the value of  x where our functon is undefined. So, we equate the bottom to zero as well. 

x = 0.

But, x = 0, is not on our interval since, we have[1/2, 2].

So, our only critical point is at x = 1.

Now, we will plug-in the endpoints of our interval and our critical points on our original function. 

`f(1/2) = (1/2) - ln(1/2) = 1.193147181 `

`f(1) = (1) - ln(1) = 1`

`f(2) = 2 - ln(2) = 1.306852819`

So, we will have an absolute minimum at x = 1, and its y = 1. Our absolute maximum will be at x = 2, which is y = 1.3069.

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