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Curve Analysis

Another valuable application of the derivative involves analyzing the graph of a given function in terms of monocity, concavity, and optimization.

  • A function \(f(x)\) is said to be monotonicity if it is either nondecreasing or nonincreasing.
  • A function \(f(x)\) is said to be nondecreasing provided that \(f(a)\leq f(b)\) where \(a<b\). If the inequality is strict i.e. \(f(a)\leq f(b)\) then the function is said to be increasing, or strictly increasing.
  • A function \(f(x)\) is said to be nonincreasing provided that \(f(a)\leq f(b)\) where \(a<b\). If the inequality is strict i.e. \(f(a)\leq f(b)\) then the function is said to be decreasing, or strictly decreasing.
  • A critical value is a value that corresponds to either a relative minimum or to a relative maximum.
  • A stationary point is a point in which the slope of the graph is zero. This can occur due to the existence of relative extrema or saddle points

A critical point is a point on a graph \(f(x)\) in which the monotonicity of the graph changes. A critical point for \(f(x)\) can be found by finding the critical values for \(f'(x)\).

In other words, if \(c\in(a,b)\) such that \((c,f(c))\) is either a relative extrema  then

  1. \(f'(c)=0\)
  2. \(f'(c)\) DNE
Monotonicty and the First Derivative
  • If \(f'(x)<0\) then \(f(x)\) is  decreasing on \((a,b)\).
  • If \(f'(x)>0\) then \(f(x)\) is  increasing on \((a,b)\).


If the multiplicity of \(c\) is even then there is no change in the monotonicity at \(x=c\).

An inflection point is a point on a graph \(f(x)\) in which the concavity of the graph changes. An inflection point for \(f(x)\) can be found by finding the critical values for \(f'(x)\).

In other words, if \(c\in(a,b)\) such that \((c,f(c))\) is an inflection point then one of the following must hold true. 

  1. \(f”(c)=0\)
  2. \(f”(c)\) DNE
Concavity and the Second Derivative 
  • If \(f”(x)<0\) then \(f(x)\) is a concave downwards on \((a,b)\).
  • If \(f”(x)>0\) then \(f(x)\) is a concave upwards on \((a,b)\).

Observe that this analysis upon the second derivative is merely the first derivative test applied to the first derivative of \(f(x)\)


If the multiplicity of \(c\) is even then there is no change in the concavity at \(x=c\).