Mean Value Theorem and finding open intervals

Violagirl

Junior Member
Joined
Mar 9, 2008
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Hi, I am not sure how to go about doing these problems and was wondering if someone could show/tell me how to start them? Thanks!

1) Determine whether the Mean Value Theorem can be applied to f on the closed interval [a,b]. If the Mean Value Theorem can be applied, find all values of c in the open interval (a,b) such that f'(c) = f(b)-f(a)/b-a.

f(x) = x log[sub:lnmqh1ob]2[/sub:lnmqh1ob]x, [1,2]

2) Use the Intermediate Value Theorem and Rolle's Theorem to prove that the equation has exactly one real solution.

x[sup:lnmqh1ob]5[/sup:lnmqh1ob]+x[sup:lnmqh1ob]3[/sup:lnmqh1ob]+x+1=0

3. Find a function f that has the derivative f'(x) and whose graph passes through the given point.

f'(x) = 2x, (1,0)


On this one, I did not know if it was possible to do without a calculator or not and was uncertain how to determine the critical points.


4. Identify the open intervals on which the function is increasing or decreasing.



y = x-2 cos x, 0 < x < 2 TT

My work:


y'=1+2 sin x

y'(0) = sin x = -1/2

Critical points: 7 TT/6, 11 TT/6

First question: Why would you pick these points as the critical points? And why 2 TT/3 and 4 TT/3 wouldn't work?

Second question: When determining which intervals are increasing and decreasing, is it possible to figure out which is which without the use of a calculator?

One point I used for the interval of 7 TT/6 < x < 11 TT/6 was 4 TT/3. I calculated each to compare each value in decimals and found 7 TT/6 = 3.67, 11 TT/6 = 5.76 and 4 TT/3 = 4.19, which is how I figured I could use 4 TT/3 to plug back into the derived equation to find a decreasing slope with the use of a calculator. But is there another way to do figure it out without a calculator? Thanks!!
 
\(\displaystyle Using \ Descartes' \ Rule \ of \ Signs, \ we \ get;\)

\(\displaystyle 2) \ f(x ) \ = \ x^{5}+x^{3}+x+1 \ \implies \ 0 \ change \ in \ sign\)

\(\displaystyle f(-x) \ = \ -x^{5}-x^{3}-x+1 \ \implies \ 1 \ change \ in \ sign\)

\(\displaystyle + - I\)
\(\displaystyle [0,1,4], \ are \ the \ only \ possible \ outcomes.\)

\(\displaystyle Therefore \ f(-1) \ = \ -2 \ and \ f(0) \ = \ 1 \ tells \ us \ (by \ the \ IVT) \ that \ there \ is \ a \ zero \ (the \ only \ zero)\)

\(\displaystyle \ between \ -2 \ and \ 1.\)

\(\displaystyle Using \ my \ trusty \ TI-89 \ f(-.636882916802) \ = \ 0, \ or \ if \ you \ prefer \ grunt \ work, \ there \ is \ always\)

\(\displaystyle Newton's \ Method \ or \ the \ method \ of \ exhaustion.\)
 
Thanks Glenn! That helped a lot! And then I was also able to figure out the others. :)
 
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