# Arc Length parametric curve

I have the following curve: $$x = \cos(t)$$ $$y = t - \sin(t)$$ $$0 \leq t \leq 2\pi$$ I have to draw the graph, point the direction and find its length. The solved the first two questions. The problem emerges when I set up the integral for the arc length. $$\frac{\partial x}{\partial t} = -\sin(t)$$ $$\frac{\partial y}{\partial t} = 1 - \cos(t)$$ $$\int ^{2\pi}_0{\sqrt{(-\sin^2t)+(1-\cos(t))^2}dt}$$ After simplification: $$\int ^{2\pi}_0{\sqrt{2-2\cdot \cos(t)}dt}$$ But how to proceed further?

• One customary approach is the trig identity $\sin^{2}(\frac{t}{2}) = \frac{1}{2}(1 - \cos t)$; see also Computing the arc length of a cycloid, both the answer and the linked external question, which includes a solution. Apr 3, 2016 at 11:10
• Thank you. I forgot this identity. It solves the task! Apr 3, 2016 at 11:14

Hint: \begin{align}\int_0^{2\pi} \sqrt {2 (1 - \cos t)} dt &= \int_0^{2\pi} \sqrt2 \sqrt{2 \sin^2 \left(\frac{t}{2} \right)} dt \\&= 2 \int_0^{2\pi}\sin \left( \frac{t}{2}\right) dt \\&= \ldots\end{align}

Split the limits into four intervals i.e $[0,\pi/2], [\pi/2,\pi], [\pi,3\pi/2] and [3\pi/2,2\pi]$.

Let $A$ = $\int ^{\pi/2}_0{\sqrt{2-2\cdot \cos(t)}dt}$ be the arc length in domain $[0,\pi/2]$.

Let $B$ = $\int ^{\pi}_{\pi/2}{\sqrt{2-2\cdot \cos(t)}dt}$ be the arc length in domain $[\pi/2,\pi]$.

Let $C$ = $\int ^{3\pi/2}_{\pi}{\sqrt{2-2\cdot \cos(t)}dt}$ be the arc length in domain $[\pi,3\pi/2]$.

Let $D$ = $\int ^{2\pi}_{3\pi/2}{\sqrt{2-2\cdot \cos(t)}dt}$ be the arc length in domain $[3\pi/2,2\pi]$.

Let $I$ = $A+B+C+D$ is the solution which is required to be calculated

Since $cos(t)$ is symmetric about the line $t-\pi=0$.This implies $\sqrt{2-2\cdot \cos(t)}$ is also symmetric about the line $t-\pi=0$.Thus $A+B=C+D$

This implies $I=2*(A+B)$

$I=2*(\int ^{\pi/2}_0{\sqrt{2-2\cdot \cos(t)}dt}+\int ^{\pi}_{\pi/2}{\sqrt{2-2\cdot \cos(t)}dt})$

$I=2*\int ^{\pi}_0{\sqrt{2-2\cdot \cos(t)}dt}$

Let $x=cos(t)$

Then $dx=-sin(t) dt$

As $sin(t)=\sqrt{1-\cos^2(t)}$

Thus $sin(t)=\sqrt{1-x^2}$

Putting the value of $cos(t)$ and $dt$ in $I$ and changing the limits we get,

$I=-2*\int ^{-1}_{1}{\sqrt{({2-2x})/({1-x^2})}dx}$

$I=2\sqrt{2}*\int ^{1}_{-1}{\sqrt{({1-x})/({1-x^2})}dx}$

$I=2\sqrt{2}*\int ^{1}_{-1}{\sqrt{1/({1+x})}dx}$

Integrating it and putting limits we get,

$I=8$ as the required answer.