[spec] add table of graphs for points of inflection

diff --git a/spec/calculus.md b/spec/calculus.md
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@@ -194,15 +194,16 @@ $$\therefore y \implies {dy \over dx} \implies {d({dy \over dx}) \over dx} \impl
 
 Order of polynomial $n$th derivative decrements each time the derivative is taken
 
-### Maxima and minima
+### Points of Inflection
 
-- if $f^\prime (a) = 0$ and $f^{\prime\prime}(a) > 0$, then point $(a, f(a))$ is a local min (curve is concave up)
+*Point of inflection* - point of maximum gradient (either +ve or -ve). Occurs where $f^{\prime\prime} = 0$
 
+- if $f^\prime (a) = 0$ and $f^{\prime\prime}(a) > 0$, then point $(a, f(a))$ is a local min (curve is concave up)
 - if $f^\prime (a) = 0$ and $f^{\prime\prime} (a) < 0$, then point $(a, f(a))$ is local max (curve is concave down)
 - if $f^{\prime\prime}(a) = 0$, then point $(a, f(a))$ is a point of inflection
 - - if also $f^\prime(a)=0$, then it is a stationary point of inflection
 
-*Point of inflection* - point of maximum gradient (either +ve or -ve)
+![](graphics/second-derivatives.png)
 
 ## Antidifferentiation
 

diff --git a/spec/graphics/second-derivatives.png b/spec/graphics/second-derivatives.png
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