Which Interval Contains the Root of the Equation?

In summary: You can use the same logic as before: f(-1)<0 and f(0)>0, so by the Intermediate Value Theorem, there must be a root in the interval [-1,0].In summary, The function xe^{x}=1 has a root in the interval [-1,0] and the function x^{3}-6x^{2}+2.826=0 has a root in the interval [-1,0]. This can be determined using the Intermediate Value Theorem.
  • #1
kathrynag
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0

Homework Statement


Find an interval of length 1 that contains a root of the equation [tex]xe^{x}=1[/tex]



Homework Equations





The Attempt at a Solution


I'm not quite sure how to find these intervals...

Homework Statement


Find an interval of length 1 that contains a root of the equation [tex]x^{3}-6x^{2}+2.826=0[/tex]


Homework Equations





The Attempt at a Solution

 
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  • #2
Do you know any method to find roots of an equation? If not you can always guess. Observe that the given functions are continuous so the intermediate value theorem applies. How dies this help you?
 
  • #3
Pere Callahan said:
Do you know any method to find roots of an equation? If not you can always guess. Observe that the given functions are continuous so the intermediate value theorem applies. How dies this help you?

So, we have f(a)<y<f(b) or f(a)>y>f(b)
So f(a)=ae^a=1
f(b)=be^b=1
 
  • #4
kathrynag said:
So, we have f(a)<y<f(b) or f(a)>y>f(b)
Erm, yes, and in particular there exist x between a and b, such that f(x)=y.
What is your f here? It is the function of which you want to find the root, so
f(x)=x ex-1

Would it help you if you could find numbers a,b with
f(a)<0 and f(b)>0 ?
 
  • #5
Pere Callahan said:
Erm, yes, and in particular there exist x between a and b, such that f(x)=y.
What is your f here? It is the function of which you want to find the root, so
f(x)=x ex-1

Would it help you if you could find numbers a,b with
f(a)<0 and f(b)>0 ?

Ok, so -1 works for a and 1 works for b. So could we have the interval [-1,1]?
 
  • #6
kathrynag said:
Ok, so -1 works for a and 1 works for b. So could we have the interval [-1,1]?
Yes, except that is not an interval of length 1! What about x= 0?
 
  • #7
Ok, so could I have [0,1] or [-0.5,0.5]?
 
  • #8
kathrynag said:
Ok, so could I have [0,1] or [-0.5,0.5]?

You should be able to verify this proposed solution yourself!

Do the intervals have lengths one?

Is f(0)<0<f(1) ?
Is f(-.5)<0<f(.5) ?

If you can answer these last question with yes, then the Intermediate Value Theorem asserts that there exists x in [0,1] or [-.5,.5] respectively with f(x)=0. This x would be the root, of course.
 
  • #9
Pere Callahan said:
You should be able to verify this proposed solution yourself!

Do the intervals have lengths one?

Is f(0)<0<f(1) ?
Is f(-.5)<0<f(.5) ?

If you can answer these last question with yes, then the Intermediate Value Theorem asserts that there exists x in [0,1] or [-.5,.5] respectively with f(x)=0. This x would be the root, of course.

Yes, it works for [-0.5,0.5], but [0,1] only works if we are letting 0<0<f(1)
 
  • #10
kathrynag said:
Yes, it works for [-0.5,0.5], but [0,1] only works if we are letting 0<0<f(1)

No it does not work for [-.5,.5]! Remember f(x)=xex-1.

What is f(-.5), f(0), f(.5), f(1)...?
 
  • #11
Pere Callahan said:
No it does not work for [-.5,.5]! Remember f(x)=xex-1.

What is f(-.5), f(0), f(.5), f(1)...?

f(-.5)=-.30326
f(0)=-1
f(.5)=-.1756
f(1)=1.71

So, [-.5,.5] does not work but [0.1] works because f(0)<1 and f(1)>1
 
  • #12
kathrynag said:
f(-.5)=-.30326
f(0)=-1
f(.5)=-.1756
f(1)=1.71

So, [-.5,.5] does not work but [0.1] works because f(0)<1 and f(1)>1

It works because f(0)<0 and f(1)>0; this means that there is a number x between zero and one with the property f(x)=0. This again means that xex-1=0 which is equivalent to xex=1.
 
  • #13
Pere Callahan said:
It works because f(0)<0 and f(1)>0; this means that there is a number x between zero and one with the property f(x)=0. This again means that xex-1=0 which is equivalent to xex=1.

Oh yeah. That's actually what I meant to type, haha.
So, now I wnat to do the same for x^3-6x^2+2.826=0
[-1,0]?
 
  • #14
kathrynag said:
So, now I wnat to do the same for x^3-6x^2+2.826=0
[-1,0]?
This is correct.
 

FAQ: Which Interval Contains the Root of the Equation?

What is an "Interval of length 1 root"?

An "Interval of length 1 root" refers to a mathematical concept in which the difference between the upper and lower bounds of an interval is equal to 1. This interval can be represented as [x,x+1], where x is any real number.

How is the length of an "Interval of length 1 root" calculated?

The length of an "Interval of length 1 root" can be calculated by subtracting the lower bound from the upper bound. For example, if the interval is [2,3], the length would be 3-2 = 1.

What is the significance of an "Interval of length 1 root" in mathematics?

"Interval of length 1 root" has various applications in calculus, probability, and number theory. It helps in understanding the concept of limits, probability distributions, and prime numbers.

Can an "Interval of length 1 root" be negative?

Yes, an "Interval of length 1 root" can be negative. For example, the interval [-2,-1] is an "Interval of length 1 root" as the difference between -2 and -1 is 1.

How is an "Interval of length 1 root" represented on a number line?

An "Interval of length 1 root" is represented on a number line by marking the lower bound and the upper bound, and then drawing a line segment connecting them. The length of the line segment will be equal to 1 unit.

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