Update Angular resolution.md

Author: eahussein

resources/links/Angular resolution.md

@@ -14,6 +14,7 @@
 It seems that in astronomy, there is a relationship between the image resolution $\theta$ and the wavelength $\lambda$ giving by
 
 $\theta = 1.22 \frac{\lambda} {D}$
+$$\int_\Omega \nabla u \cdot \nabla v~dx = \int_\Omega fv~dx$$
 
 - Where $D$ is the diameter of the telescope.
 - to get better resolution (small $\theta$), we need: