I can't see what I'm doing wrong here, it's very simple.
The length of the curve $x^2$ between $0$ and $4$ is $\int_0^4 \sqrt{(1+4x^2)}dx$ isn't it?
I don't know how to calculate this but wolframalpha tells me that it's about $16.8$ units. That can't be right: maybe it's an optical illusion but I have it drawn up on my board and the curve doesn't look any longer than 8 units.
What am I doing wrong?
EDIT: You know what? I realised my mistake: my y-axis is scaled. Doh!
Feel free to delete this question :)