Update cheatsheet.en.md

01.curriculum/01.physics-chemistry-biology/03.Niv3/02.Geometrical-optics/02.geometrical-optics-foundings/01.concept-ray-of-light/02.concept-ray-of-light-overview/cheatsheet.en.md

@@ -30,7 +30,7 @@ Light rays *do not interact with each other*
 ##### The refraction index
 
 **Refractive Index $`n`$**       
-**$`n \; = \; \frac{c}{v}`$**
+**$`n \; = \; \dfrac{c}{v}`$**
 * **`c`** : *speed of light in vacuum* (absolute limit)
 * **`v`** : *speed of light in the middle* homogeneous
 
@@ -53,7 +53,7 @@ I wanted to make this important remark here, but it is not simple: with Doppler
 !! $`n`$: complex value dependent on the $\nu$ frequency of the electromagnetic wave, strong variations representative of all light / matter interaction mechanisms: $`n (\nu) = \Re[n(\nu )] + \Im[n(\nu)]`$ <br>
 !!
 !! In the visible domain (where $`\lambda_0`$ is more used than $`\nu`$) and for transparent medium : <br>
-!! real value, small variations of $`n`$ with $`\lambda_0`$ ($`\frac{\Delta n}{n} <1 \%`$)
+!! real value, small variations of $`n`$ with $`\lambda_0`$ $`\left(\frac{\Delta n}{n} < 1\%\right)`$
 
 ##### Optical path