Adding and subtracting integrals

So, I'm really rusty with math and I have a question that is stumping me. It's making me really mad because I'm sure somewhere in the back of my mind I know how to do it, but I can't figure it out. Here is the question:

I will denote: squared as (f)^2 and integrals as int[function]. all integrals go from a to b and f''(x) = second derivative of f

Verify that for f,g in R:

int[f''(x)^2] dx - int[g''(x)^2] dx = int[(f''(x)-g''(x))^2] dx + 2 int[g''(x)(f''(x)-g''(x))]dx

I'd really appreciate some help or ideas, I'm pretty sure there is a way to do it without trying to solve the integral, and all i have to do is verify the result. thanks!

Re: Adding and subtracting integrals

So you're trying to verify that

Is that correct? If so, I'd say your starting-point would be to note that

assuming all three integrals exist. What does this enable you to do?

Re: Adding and subtracting integrals

In my opinion it's just a fact of using some algebraic rules.

If you want to prove it: RHS -> LHS:

Simplify ...

Re: Adding and subtracting integrals

Quote:

Originally Posted by

**seamstress** S

Verify that for f,g in R:

int[f''(x)^2] dx - int[g''(x)^2] dx = int[(f''(x)-g''(x))^2] dx + 2 int[g''(x)(f''(x)-g''(x))]dx

I think that you have entered some terms incorrectly.

Should it be

Re: Adding and subtracting integrals

no, the two up there had it. thanks though!

Re: Adding and subtracting integrals

yeah, now i really feel like an idiot...that was super easy. guess i really am rusty. thank you

Re: Adding and subtracting integrals

however, i still can't figure out how to integrate the equation by parts. any pointers? thanks

Re: Adding and subtracting integrals

NEVERMIND i got it. just please don't answer any more of my questions today- it's been an off one.