more consistent use of details/summary

mechanical/mmme1026_maths_for_engineering/calculus.md

@@ -40,6 +40,9 @@ Its range has to exclude all those values of $x$ where $g(x) = 0$.
 
 </details>
 
+<details>
+<summary>
+
 ### Inverse Functions
 
 Consider the function $f(x) = y$.
@@ -48,6 +51,12 @@ we can define the inverse $f^{-1}$ as:
 
 $$f^{-1}(y) = f^{-1}(f(x)) = x$$
 
+</summary>
+</details>
+
+<details>
+<summary>
+
 ### Limits
 
 Consider the following:
@@ -64,6 +73,8 @@ $$\lim_{x \rightarrow a} f(x)$$
 
 to be the limiting value, if it exists, of $f(x)$ as $x$ gets approaches $a$.
 
+</summary>
+
 #### Limits from Above and Below
 
 Sometimes approaching 0 with small positive values of $x$ gives you a different limit from
@@ -79,12 +90,16 @@ $$\lim_{x \rightarrow a^-} f(x)$$
 
 If the two limits are equal, we simply refer to the *limit*.
 
+
+</details>
+
 ## Important Functions
 
 ### Exponential Function
 
 $$f(x) = e^x = \exp x$$
 
+
 <details>
 <summary>
 It can also be written as an infinite series: