Hey all

I do Calculas of Variations, and have an exam in around nine hours, so would appreciate help before then. After is good too thhough.

I got a solution, but it has a bit I don't understand, was hoping you guys could help.

My problem is:

Find extremal of J(y)= ,subject to .

Solution that I have gotten is:

Integrate:

C arbitary constant

so

Put u in, integrate, change the result around a small bit and you get

D arbitary constant

inset bound conditions, you get

So,

Diffretiate with respect to x;

Then (And herin lies the problem), I get

How does he change into

Thanks in advance.