What is the area of the set $\frac{x^2}{a^2} + \frac{y^2}{b^2} \leq 1$? 
Problem: Let $a, b$ be positive real constants. Calculate the area of the set $$\mathcal{E} = \left\{ (x,y) \in \mathbb{R}^2 : \frac{x^2}{a^2} + \frac{y^2}{b^2} \leq 1 \right\}$$

I am able to approach the problem with logical thinking like that:

For $a=b=1$ the set describes an unit circle. We can write the
  condition as  $$ \left(\frac{x}{a}\right)^2 +
\left(\frac{y}{b}\right)^2 $$ which makes clear that the constants
  $a,b$ stretch the circle in $x,y$ direction. Thus we have an ellipse
  with the axis $a$ and $b$. The equation for the area of an ellipse is
  know to be $A = \pi a b$.

However, I guess that I am supposed to apprach the problem with some more general method (e.g. integrals, etc.). So how would I solve this problem in a more formal way? E.g. for sets with conditions like that in general?
 A: The area that you want to compute is$$2b\int_{-a}^a\sqrt{1-\frac{x^2}{a^2}}\,\mathrm dx.$$Doing the substitution $x=at$ and $\mathrm dx=a\,\mathrm dt$, you get the integral$$2ab\int_{-1}^1\sqrt{1-t^2}\,\mathrm dt.$$But a primitive of $\sqrt{1-t^2}$ is $\frac12\left(t\sqrt{1-t^2}+\arcsin t\right)$. So,$$\int_{-1}^1\sqrt{1-t^2}\,\mathrm dt=\frac12\left(\arcsin(1)-\arcsin(-1)\right)=\frac\pi2$$and therefore your area is $\pi ab$, as you guessed.
A: The area is
$$
\iint\limits_{\mathcal{E}}dx\,dy
$$
With the substitution $x=a\rho\cos\varphi$, $y=b\rho\sin\varphi$ (with $\rho\ge0$ and $0\le\varphi<2\pi$), the limitation on $x$ and $y$ for $(x,y)\in\mathcal{E}$ become
$$
\frac{a^2\rho^2\cos^2\varphi}{a^2}+\frac{b^2\rho^2\sin^2\varphi}{b^2}\le 1
$$
that is, $\rho^2\le1$ and so $0\le\rho\le1$; no further limitation on $\varphi$ is implied. 
Why that substitution? Because we do know the set $\mathcal{E}$ is an ellipse, don't we?
The Jacobian is $ab\rho$, thus the integral becomes
$$
\iint\limits_{\substack{\rho\in[0,1]\\\varphi\in[0,2\pi]}}
  ab\rho\,d\rho\,d\varphi
=
ab\int_0^{2\pi}\biggl(\int_0^1 \rho\,d\rho\biggr)\,d\varphi=\pi ab
$$
A: Parametrize: $x=a \cos(\theta)$,  $ y= b\sin(\theta),$  $0\le \theta \lt 2π.$
1st quadrant:
$\int_{0}^{a} ydx =$
$\int_{π/2}^{0} b\sin(\theta)(-a\sin(\theta))d\theta=$
$-ab\int_{π/2}^{0}\sin^2(\theta)d\theta=$
$ab\int_{0}^{π/2}\sin^2(\theta)d\theta.$
Note:
$ \int_{0}^{π/2}\sin^2(\theta)d\theta =$
$\int_{0}^{π/2}\cos^2(\theta)d\theta.$
Also: 
$\sin^2(\theta)+\cos^2(\theta)=1.$
Now integrating LHS and RHS from $0$ to $π/2:$
$\rightarrow:$
$\int_{0}^{π/2}\sin^2(\theta)d\theta = π/4.$
Finally:
Area of the ellipse : $4 (π/4)ab =πab.$
