Update textbook.fr.md

12.temporary_ins/96.electromagnetism-in-media/20.reflexion-refraction-at-interfaces/20.metallic-waveguides/10.main/textbook.fr.md

@@ -27,7 +27,7 @@ _TE and TM waves and their corresponding standing wave behaviour along the y axi
 _equations_
 
 $`\overrightarrow{E}_{\perp}=\underbrace{-2\,E_0\,\sin(k\cdot y\cdot \cos\,\theta)}_{\large{amplitude}}
-\times \underbrace{\sin\big((k\,\sin\,\theta}_{wavevector}}\,z-\omega t\big)\overrightarrow{e_z}`$
+\times big(\underbrace{\sin\k\,\sin\,\theta}_{\large{wavevector}}\,z-\omega t\big)\overrightarrow{e_z}`$
 
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 []{#_bookmark79 .anchor}$`\overrightarrow{E}`$~⊥~ = −2*E*~0~ sin (*k y* cos *θ*) sin