diff --git a/Electrostatics/microstrip.pro b/Electrostatics/microstrip.pro
index 444ea15525c898eaa13ca9a61c91f86bb8a5fe50..27fee8211cbce18252da9b31ad9cec4cb9e1baf6 100644
--- a/Electrostatics/microstrip.pro
+++ b/Electrostatics/microstrip.pro
@@ -190,7 +190,7 @@ Formulation {
      product over a domain D. If the test-functions v' are differentiable,
      integration by parts using Green's identity leads to finding v such that
 
-     (epsilon grad v, grad v')_Vol_Ele + (n . (epsilon grad v), v')_Bnd_Vol_Ele = 0
+     (epsilon grad v, grad v')_Vol_Ele - (n . (epsilon grad v), v')_Bnd_Vol_Ele = 0
 
      holds for all v', where Bnd_Vol_Ele is the boundary of Vol_Ele. In our
      microstrip example this surface term vanishes, as there is either no test