Hey there! Sign in to join this conversationNew here? Join for free
 You are Here: Home >< Maths

# C3 help please? Watch

1. Hi im doing the review exercises for C3 in my text book, and the question is 'show that the equation x=ln(x+5) has a root between x=1 and x=2'. i just cant seem to do it :/ help would be much appreciated!
2. (Original post by noodlemon)
Hi im doing the review exercises for C3 in my text book, and the question is 'show that the equation x=ln(x+5) has a root between x=1 and x=2'. i just cant seem to do it :/ help would be much appreciated!
What have you tried?
3. (Original post by noodlemon)
Hi im doing the review exercises for C3 in my text book, and the question is 'show that the equation x=ln(x+5) has a root between x=1 and x=2'. i just cant seem to do it :/ help would be much appreciated!
Hint:

Let

and notice that if, at x=a, f(x) crosses the x axis, it must be above on one side of a and below on the other.
4. (Original post by TenOfThem)
What have you tried?
ok ive tried getting rid of the ln getting x+5=e^x? i suppose you need to make x the subject now right?
5. (Original post by noodlemon)
ok ive tried getting rid of the ln getting x+5=e^x? i suppose you need to make x the subject now right?
Nope

Did you try putting 1 and 2 in to see what happens
6. (Original post by Indeterminate)
Hint:

Let

and notice that if, at x=a, f(x) crosses the x axis, it must be above on one side of a and below on the other.
so are you saying to use trial and error?
7. (Original post by noodlemon)
so are you saying to use trial and error?
You are not trying to find x

RTQ
8. (Original post by TenOfThem)
You are not trying to find x

RTQ
Oh i see now, sorry ah good advice.. i subbed in 1 to the equation and got 0.7917... and 2 to get -0.054.. meaning it must have crossed the x axis between those points of 1 and 2. thanks
9. (Original post by noodlemon)
so are you saying to use trial and error?
Well, no.

Thus if you let f(x) equal the LHS, you need to show that there is a root of f(x)=0 between 1 and 2 (i.e it crosses the x axis)

Use the fact that, to cross the x-axis, it must be above the x axis at one point (on one side of the root), and below on the other.

See if you can relate what I'm saying to the values of f(x) when you substitute 1 and 2 in
10. (Original post by Indeterminate)
Well, no.

Thus if you let f(x) equal the LHS, you need to show that there is a root of f(x)=0 between 1 and 2 (i.e it crosses the x axis)

Use the fact that, to cross the x-axis, it must be above the x axis at one point (on one side of the root), and below on the other.

See if you can relate what I'm saying to the values of f(x) when you substitute 1 and 2 in
yup aha thanks guys i was overcomplicating the question and trying to find the exact value of x :3

Reply
Submit reply
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: March 24, 2013
Today on TSR

### 'Entry requirements are a form of elitism'

What do you think?

### Roommate kissed my boyfriend

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• Poll
Useful resources

## Make your revision easier

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams

Can you help? Study help unanswered threads

## Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.