<s>[INST] Is it true that $\iint_{x^2+y^2 \le 1} f(ax+by+c) dx dy=2 \int_{-1}^1 \sqrt{1-u^2}f(u\sqrt{a^2+b^2} +c)du $? Is it true that $\iint_S f(ax+by+c) dx dy=2 \int_{-1}^1 \sqrt{1-u^2}f(u\sqrt{a^2+b^2} +c)du $  ? 
where $S:=\{(x,y) \in \mathbb R^2 : x^2+y^2 \le 1\}$  
I know I need to do a change of variable , but I don't know what change of variable .
Please help . Thanks in advance 
 [/INST] Indeed, it is true. Equating the arguments of $f$ we obtain
\begin{align*}
ax+by+c&=u\sqrt{a^2+b^2}+c\\
\frac{ax+by}{\sqrt{a^2+b^2}}&=u
\end{align*}

This suggests a transformation by rotation according to the comment of @achillehui
  \begin{align*}
u=\frac{ax+by}{\sqrt{a^2+b^2}}\qquad&\qquad x=\frac{au-bv}{\sqrt{a^2+b^2}}\\
\tag{1}\\
v=\frac{-bx+ay}{\sqrt{a^2+b^2}}\qquad&\qquad y=\frac{bu+av}{\sqrt{a^2+b^2}}
\end{align*}

In order to transform the integral
\begin{align*}
\iint_{x^2+y^2\leq 1}f(ax+by+c)\,dx\,dy
\end{align*}
we first consider the transformation of the disc $x^2+y^2\leq 1$. Since the transformation is a rotation we expect as result the unit disc in the variables $u$ and $v$.

We obtain using (1)
  \begin{align*}
x^2+y^2&=
\frac{(au-bv)^2}{a^2+b^2}+\frac{(bu+av)^2}{a^2+b^2}\\
&=\frac{(a^2u^2-2abuv+b^2v^2)+(b^2u^2+2abuv+a^2v^2)}{a^2+b^2}\\
&=u^2+v^2
\end{align*}

We need the Jacobian determinant
$\frac{\partial(x,u)}{\partial(u,v)}$
in order to transform the integral:
\begin{align*}
\frac{\partial(x,u)}{\partial(u,v)}&=\begin{vmatrix}x_u&x_v\\y_u&y_v\end{vmatrix}
=\begin{vmatrix}
\frac{a}{\sqrt{a^2+b^2}}&\frac{-b}{\sqrt{a^2+b^2}}\\
\frac{b}{\sqrt{a^2+b^2}}&\frac{a}{\sqrt{a^2+b^2}}\\
\end{vmatrix}\\
&=\frac{1}{a^2+b^2}\begin{vmatrix}a&-b\\b&a\end{vmatrix}\\
&=1
\end{align*}

We obtain
  \begin{align*}
\iint_{x^2+y^2\leq 1}&f(ax+by+c)\,dx\,dy=\\
&=\iint_{u^2+v^2\leq 1}f(u\sqrt{a^2+b^2}+c)\frac{\partial(x,u)}{\partial(u,v)}\,du\,dv\\
&=\int_{-1}^{1}\int_{-\sqrt{1-u^2}}^{\sqrt{1-u^2}}f(u\sqrt{a^2+b^2}+c)\,dv\,du\\
&=\int_{-1}^{1}f(u\sqrt{a^2+b^2}+c)\left(\int_{-\sqrt{1-u^2}}^{\sqrt{1-u^2}}1\,dv\right)\,du\\
&=2\int_{-1}^{1}\sqrt{1-u^2}f(u\sqrt{a^2+b^2}+c)\,du\\
\end{align*}

</s>