<a name="gouttiere">La gouttire  (verticale) d'quations  \(x^2 + y^2 = 1), \(z_0 \leq z \leq z_1) est d'quations paramtriques 

<p><center>\( \left \lbrace
\begin{matrix} x=&\cos(\theta)\\
y=&\sin(\theta)\\z=&z\end{matrix} \quad (\theta,\varphi)\in [\theta_0,2\pi-\theta_0] \times [z_1,z_2])</center></p>

<table align="center"><tr><td>
\draw{200,250}
{xrange -1, 3
yrange -1,2.5
hline 0,0,black
vline -0.5,0,black
linewidth 2
lines red, 0,1,2,1,2,2,0,2,0,1
arrow 0,1,1,1,6,red
arrow 2,1,2,1.5,6,red
arrow 2,2,1.5,2,6,red
arrow 0,2,0,1.5,6,red
fill 0.5,1.5,skyblue
copy 2.5,0.5,-1,-1,-1,-1,mathfonts/109/theta.gif
text black , -0.5,2, medium,z 
text black, 0,1,medium, a
text black, 2,1,medium, b
text black, 2,2,medium, c
text black, 0,2,medium, d

}
</td><td> \(\longrightarrow)</td><td>\draw{300,200}
{xrange -3.5,4
yrange -3.5,4
line 0.5,-3.2,0.5,1.8,red
line -0.5,-3.2,-0.5,1.8,red
line -0.9,-2.7,-0.9,2.1,skyblue
line 0.9,-2.7,0.9,2.1,skyblue
arc 0,2.5,2,2, 300,240,red
arc 0,-2.5,2,2, 300,240,red
text black,0.9,-2.7,medium,A
text black,0.9,2.1,medium,B
text black,-0.9,2.1,medium,C
text black,-0.9,-2.7,medium,D
arrow 0.5,-0.5,0.5,1,6,black
arrow -0.5,1,-0.5,-0.5,6,black
arrow 0.2,3.5,-0.2,3.5,6,black
}
</td></tr>
</table>