# What is the arc length of #f(x) = 3xln(x^2) # on #x in [1,3] #?

##### 1 Answer

19.88

#### Explanation:

First, we can observe that

For future use, we can calculate its derivative using product rule, getting

We now consider an infinitesimal arclength,

We can actually factor out a common factor

and now

Now, we can integrate! Let the total arc length be

Now this integral looks very ugly. And it is! This doesn't have an analytical (exact) answer, as far as I can tell.

We can't prove that this doesn't have a clean answer easily. I proved it to myself by using u-subsitution with u = ln(x) and searching an integration table for integrals of functions of the square root of quadratics times an exponential with no luck.

Anyway, that means we just have to plug this into a calculator to get an answer: