diff --git a/docs/7_tight_binding.md b/docs/7_tight_binding.md
index ae98626ec496f7716375e8ef1af3b18bbcec4a2c..e73cc838bd69f1356a309f80a3475a3d19382d6e 100644
--- a/docs/7_tight_binding.md
+++ b/docs/7_tight_binding.md
@@ -354,7 +354,7 @@ $$ E = E_0 - 2t \cos(ka), $$
 we get
 
 $$
-ka = \pm\arccos[(E - E_0) / 2t],
+ka = \pm\arccos[-(E - E_0) / 2t],
 $$
 
 and