fix header levels

mechanical/mmme1026_maths_for_engineering/calculus.md

@@ -95,16 +95,16 @@ If the two limits are equal, we simply refer to the *limit*.
 
 ## Important Functions
 
+<details>
+<summary>
 ### Exponential Function
 
 $$f(x) = e^x = \exp x$$
 
-
-<details>
-<summary>
-It can also be written as an infinite series:
 </summary>
 
+It can also be written as an infinite series:
+
 $$\exp x = e^x = 1 + x + \frac{x^2}{2!} + \frac{x^3}{3!} + ...$$
 
 </details>
@@ -116,6 +116,11 @@ The two important limits to know are:
 
 Note that $e^x > 0$ for all real values of $x$.
 
+</details>
+
+<details>
+<summary>
+
 ### Hyperbolic Functions (sinh and cosh)
 
 The hyperbolic sine ($\sinh$) and hyperbolic cosine function ($\cosh$) are defined by:
@@ -123,6 +128,8 @@ The hyperbolic sine ($\sinh$) and hyperbolic cosine function ($\cosh$) are defin
 $$\sinh x = \frac 1 2 (e^x - e^{-x}) \text{ and } \cosh x = \frac 1 2 (e^x + e^{-x})$$
 $$\tanh = \frac{\sinh x}{\cosh x}$$
 
+</summary>
+
 ![[Fylwind at English Wikipedia, Public domain, via Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Sinh_cosh_tanh.svg)](./images/Sinh_cosh_tanh.svg)
 
 Some key facts about these functions:
@@ -136,17 +143,33 @@ Some key facts about these functions:
   - $\frac{\mathrm{d}}{\mathrm{d}x} \cosh x = \sinh x$
   - $\frac{\mathrm{d}}{\mathrm{d}x} \tanh x = \frac{1}{\cosh^2x}$
 
-## Natural Logarithm
+</details>
+
+<details>
+<summary>
+
+### Natural Logarithm
 
 $$\ln{e^y} = \ln{\exp y} = y$$
 
+</summary>
+
 Since the exponential of any real number is positive, the domain of $\ln$ is $x > 0$.
 
-## Implicit Functions
+</details>
+
+<details>
+<summary>
+
+### Implicit Functions
 
 An implicit function takes the form
 
 $$f(x, y) = 0$$
 
+</summary>
+
 To draw the curve of an implicit function you have to rewrite it in the form $y = f(x)$.
 There may be more than one $y$ value for each $x$ value.
+
+</details>