You are Here: Home >< Maths

# C3 Edexcel Do I state Change of Sign AND continuous??

Announcements Posted on
Last day to win £100 of Amazon vouchers - don't miss out! Take our quick survey to enter 24-10-2016
1. when doing numerical methods questions there is often a part that wants you to prove that there's a root at a particular x-value. To do this I know I have to find the change of sign in y-values for an interval between that x-value and then state that because there is a change in sign AND F(X) IS CONTINUOUS, there is a root at that x-value. My issue is not knowing how f(x) is continuous, and whether saying its continuous applies to all such questions? Any help would be much appreciated.
2. (Original post by lordoftheties)
when doing numerical methods questions there is often a part that wants you to prove that there's a root at a particular x-value. To do this I know I have to find the change of sign in y-values for an interval between that x-value and then state that because there is a change in sign AND F(X) IS CONTINUOUS, there is a root at that x-value. My issue is not knowing how f(x) is continuous, and whether saying its continuous applies to all such questions? Any help would be much appreciated.
Just substitute the x value in. If it's a root you get 0.
3. (Original post by morgan8002)
Just substitute the x value in. If it's a root you get 0.
That's the other type of question. Im referring to this kind (part c):
4. (Original post by lordoftheties)
That's the other type of question. Im referring to this kind (part c):
Yes, you'd need to state continuity. Exponentials, sines, cosines, constant functions and polynomials are all continuous. Also, finitely adding, multiplying or composing continuous functions gives a continuous function so g(x) is continuous.
5. Yes just always say "sign change, therefore root in the interval, given that f(x) is continuous"
6. (Original post by 雷尼克)
Yes just always say "sign change, therefore root in the interval, given that f(x) is continuous"
Im not sure if youre ALWAYS supposed to say continuous, the bold section of this part of the mark scheme for that question says something about not stating continuous...
7. (Original post by lordoftheties)
Im not sure if youre ALWAYS supposed to say continuous, the bold section of this part of the mark scheme for that question says something about not stating continuous...
That has nothing to do with 'continuous'... that part of the marking scheme is telling the examiner that it is an invalid method to continue the iterations.
8. (Original post by lordoftheties)
Im not sure if youre ALWAYS supposed to say continuous, the bold section of this part of the mark scheme for that question says something about not stating continuous...
It's not talking about continuity. It's talking about substituting repeatedly into the difference equation from (b) to find bounds on .
9. (Original post by morgan8002)
It's not talking about continuity. It's talking about substituting repeatedly into the difference equation from (b) to find bounds on .
Oh I see now. Thanks a lot guys.
10. (Original post by lordoftheties)
when doing numerical methods questions there is often a part that wants you to prove that there's a root at a particular x-value. To do this I know I have to find the change of sign in y-values for an interval between that x-value and then state that because there is a change in sign AND F(X) IS CONTINUOUS, there is a root at that x-value. My issue is not knowing how f(x) is continuous, and whether saying its continuous applies to all such questions? Any help would be much appreciated.
The function does have to be contiuous for this logic to work, but Edexcel don't expect you to say it.

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: May 3, 2016
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:
Today on TSR

### How does exam reform affect you?

From GCSE to A level, it's all changing

Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read here first

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams