# Finding an area of parametric curve that lies above and below x axis.

Find an area from $$t \in <0, 2\pi>$$ bounded by parametric curve:

$$\begin{cases} x(t) = \cos^3{t} \\ y(t) = \sin^3{t}\end{cases}$$

I believe the formula is: area $$=\int^{2\pi}_{0}y(t)x'(t)dt$$.

$$x'(t)=-3\cos^2t\sin{t}$$

$$\Rightarrow -3\int^{2\pi}_{0} \sin^3t\cos^2t\sin{t}dt = \frac{1}{64}(-12t + 3\sin{(2t)} + 3\sin{(4t)} - \sin{(6t)}) = \frac{-3\pi}{16}$$

I calculated the integral with wolfram as it's sort of complicated.

Area inside curve given by parametric equation

I found this thread but I don't understand why they're putting minus in front of the integral from the very beginning.

This is how the curve looks like: My questions:

1. Why is there a minus in front of integral from the link I posted?

2. How should I figure out on my own that the graph looks like that (part of it lies below x axis and part above), and what should I do with that information? Is there any general rule to follow for all kinds of "simple" parametric curves?

3. Perhaps the most important: is there any online calculator that can calculate an area of parametric curves? At the moment I don't know how to check if my answers are correct.

I can give answers for questions $$1$$ and $$2$$:

1. There's a minus sign because, if $$x(t)=\cos^3t$$, by the chain rule, $$\; x'(t)=3\cos^2t\times (-\sin t)=-3\cos^2\sin t$$.
2. You can imagine the symmetries of the curve using the properties of the trigonometric functions:

$$x(t+\pi=-x(t)$$, $$\;y(t+\pi)=-y(t)$$, so the curve has a symmetry w.r.t. the origin.

$$x(-t)=x(t)$$, $$\;y(-t)=-y(t)$$ so the curve has a symmetry w.r.t. the $$x$$-axis.

$$x(\pi-t)=-x(t)$$, $$\;y(\pi -t)=y(t)$$: the curve has a symmetry w.r.t. the $$y$$-axis.

$$x(\frac \pi 2-t)=y(t)$$, $$\;y(\frac \pi 2-t)=x(t)$$: the curve has a symmetry w.r.t. the line $$y=x$$.

As the parameterisation has period $$2\pi$$, it shows the area is $$\int_0^{2\pi}\sin^3t(-3\cos^2t\sin t)\,\mathrm dt=4\int_0^{\frac\pi 2}\sin^3t(-3\cos^2t\sin t)\,\mathrm dt=-12\int_0^{\frac\pi 2}\sin^4t\cos^2t\,\mathrm dt.$$ Note the latter integral can easily computed by linearisation of the trigonometric monomial $$\sin^4t\cos^2t$$.

• About the minus. math.stackexchange.com/a/889524/617563 There are two minuses: one from the chain rule, and one from the very beginning, even before applying chain rule. I don't know why there's minus from the very beginning. – weno Apr 10 at 15:31
• I've taken a look at this question. I think this is because $x(t)$ is here a decreasing function of $t$ between $0$ and $\pi$, so the integral of a positive function will be negative (you might in my formulæ consider the integral from $\frac \pi 2$ to $0$ instead. – Bernard Apr 10 at 15:37