You are Here: Home >< Maths

# Integration help in the form lnf(x)+c...... watch

1. Substitution u = x^2-4
2. (Original post by DFranklin)
Substitution u = x^2-4
Thanks so much but can you not do it this way: ?
I understand the substitution way by the way.

EDIT : Sorry I asked a silly question here You can't always use a certain integration technique. Thanks so much for the help
3. (Original post by sienna2266)
Thanks so much but can you not do it this way: ?
No - the solution doesn't involve ln, so your formula can't possibly apply.

I understand the substitution way by the way.
Note that your f'(x)/f(x) example is merely the same thing as applying the substitution u = f(x); the f'(x) is divided out and you are left with , which comes to ln|u| = ln|f(x)|.

Try integrating using the substitution u = x^2-4; you should be able to see how this isn't really any different from the formula you're using, it's just there are some steps your formula 'skips' out.

The point is that you can use the same approach (if not formula) for *any* scenario where you have and you know how to integrate .

More explictly, if , then

In this case, you have , rather than in the formula you've quoted. But g is still easy to integrate, so you can do this all mentally with practice.
4. (Original post by DFranklin)
No - the solution doesn't involve ln, so your formula can't possibly apply.

Note that your f'(x)/f(x) example is merely the same thing as applying the substitution u = f(x); the f'(x) is divided out and you are left with , which comes to ln|u| = ln|f(x)|.

Try integrating using the substitution u = x^2-4; you should be able to see how this isn't really any different from the formula you're using, it's just there are some steps your formula 'skips' out.

The point is that you can use the same approach (if not formula) for *any* scenario where you have and you know how to integrate .

More explictly, if , then

In this case, you have , rather than in the formula you've quoted. But g is still easy to integrate, so you can do this all mentally with practice.
Thank you so much!

### 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: February 12, 2018
Today on TSR

### He lied about his age

Thought he was 19... really he's 14

### University open days

Wed, 25 Jul '18
2. University of Buckingham
Wed, 25 Jul '18
3. Bournemouth University
Wed, 1 Aug '18
Poll
Useful resources

### 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