Evaluate the integral by interpreting it in terms of areas.

37.

the answer is

Where .. the heck did come from?

I was trying to do

and then as

then plugging into then solving for the limit. It came out to be a super complicated root problem so I look in the back to see if I'm in the right track.. and theres a in the answer...

I think I'm missing something here... how is this integral end up having a ?