# Thread: Arc Length of Polynomials

1. ## Arc Length of Polynomials

I need to find the arc length of ((x^2)/2 -ln(x)/4) from 2 to 4.

Using the arc length formula, I get that L = ∫ds = ∫ sqrt(1+(f'(x))^2) dx =

∫ sqrt (1 + ((x^3)/6 -1/(4x))^2) dx from x=2 to x=4,

but I cannot solve it any further.

I cannot use u-substitution and I have tried using trig-substitution but it does not seem to work either.

The answer given by WolframAlpha is (1/4)(24 +ln(2)) which is correct, but I want to know how it computed that. I understand that the ln(2) part came from ln(4) - ln(2) = ln(4/2) but beyond that I am lost.

2. ## Re: Arc Length of Polynomials

If \displaystyle \begin{align*} f(x) = \frac{x^2}{2} - \frac{\ln{(x)}}{4} \end{align*} then \displaystyle \begin{align*} f'(x) = x - \frac{1}{4x} \end{align*}. It looks like you integrated instead of differentiating...