# Arclength Problem; stuck on integration

1. Jul 3, 2016

### hsbhsb

1. The problem statement, all variables and given/known data
Find the arclength of $$r(t) = <{t}^2/2,{t}^3/3>$$ from t=-1 to t=1

2. Relevant equations

I have used this equation for arclength $$\int_{-1}^{1}|{r}'(t)|dt$$

3. The attempt at a solution

After integrating (using u substitution) I have the solution

$$\frac{1}{3}({t^2+1})^\frac{3}{2}$$ evaluated from t = -1 to t = 1. Of course, this yields 0! Which doesn't make any sense for arclength.

I know that the problem is somewhere in my integration step because in mathematica both the ArcLength[ $$\int_{-1}^{1}|{r}'(t)|dt$$] function and the Integrate[$$\sqrt{t^2+t^4}$$] give the same result $$\frac{2}{3}(-1+2\sqrt{2})$$, which seems to be the solution. What am I doing wrong?

2. Jul 3, 2016

### SteamKing

Staff Emeritus
You've made some kind of mistake in your original calculation, but you don't provide your work, so the details elude us.

I would go back and carefully check the integration, including any u-subs you made.

3. Jul 4, 2016

### ehild

I quess you forgot that $$\sqrt{t^2+t^4}=|t|\sqrt{1+t^2}$$.

4. Jul 4, 2016

### Ray Vickson

You are making a standard error, but rather than telling you exactly what it is I will attempt to lead you do discover it for yourself---that will server you best in the long run.

Draw a graph of $f(t) = \sqrt{t^2+t^4}$ over $-1 \leq t \leq 1$. Can you see the meaning of the integral $\int_{-1}^1 f(t) \, dt$?
For an even function $f(t)$ we have $\int_{-a}^a f(t) \, dt = 2 \int_0^a f(t) \, dt$. Using that is the best way to avoid elementary blunders.