Turn on thread page Beta
 You are Here: Home >< Maths

# annoying P5 algebra! watch

1. been doing this question for 15 miin- to half and hour and its really starting to bug me - help!!!

y=arsinh x, show that

c) (1+x^2)f'''(x)+3x.f''(x)+f'x) = 0 - looks trivial, but its really long and cant get the LHS to equal teh zero after the differentiation and resubbing it

(1+x^2) [f''(x)]^2 = 1 (this is given so use it

pk
2. (Original post by Phil23)
been doing this question for 15 miin- to half and hour and its really starting to bug me - help!!!

y=arsinh x, show that

c) (1+x^2)f'''(x)+3x.f''(x)+f'x) = 0 - looks trivial, but its really long and cant get the LHS to equal teh zero after the differentiation and resubbing it

(1+x^2) [f''(x)]^2 = 1 (this is given so use it

pk
I think your hint is wrong - don't you mean

(1+x^2) [f'(x)]^2 = 1

If you differentiate this you get

(1+x^2) 2 f' f'' + 2x (f')^2 = 0

Divide by 2 f' =/= 0 to get

(1+x^2) f'' + x f' = 0

The differentiate this to get

(1+x^2) f''' + 2x f'' + x f'' + f' = 0

which is what you want

Turn on thread page Beta

### Related university courses

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: June 18, 2005
The home of Results and Clearing

### 2,844

people online now

### 1,567,000

students helped last year
Today on TSR

### Took GCSEs this summer?

Fill in our short survey for Amazon vouchers!

### University open days

1. Bournemouth University
Clearing Open Day Undergraduate
Wed, 22 Aug '18
2. University of Buckingham
Thu, 23 Aug '18
3. University of Glasgow
Tue, 28 Aug '18
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

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