# Homework Help: Understanding Arc Length

1. Dec 19, 2011

### Screen

1. The problem statement, all variables and given/known data

This is probably very simple, but I'm teaching myself arc length via Paul's Online Calculus Notes and there's a simplification on the page:

I was wondering why the first $Δx^2$ was simplified to 1? I understand the other $Δx^2$ came out of the square root.

2. Dec 19, 2011

### vela

Staff Emeritus
What you're thinking is akin to $\sqrt{a^2+b^2} = \sqrt{a^2+1}\ b$, which isn't correct. You can only pull something out of a radical if it's a factor. What Paul is doing is this:
$$\sqrt{\Delta x^2 + [f'(x_i^*)]^2 \Delta x^2} = \sqrt{\Delta x^2(1 + [f'(x_i^*)]^2)}$$Now because Δx2 is a factor under the radical, you can say
$$\sqrt{\Delta x^2(1 + [f'(x_i^*)]^2)} = \sqrt{\Delta x^2}\sqrt{1 + [f'(x_i^*)]^2}= \Delta x \sqrt{1 + [f'(x_i^*)]^2}$$

3. Dec 19, 2011

### Screen

Oh you are completely right, sorry for such a silly question and thank you for explaining.