Critical Points#
Definition
A critical point of the function \(f\) is any number \(c\) in the domain of \(f\) such that
Critical points of \(f\) correspond to possible locations of relative extrema.
Example 1#
Find all critical points of \(f(x)=2x^3-15x^2+36x+20\).
Step 1: Compute \(f'(x)\).
Step 2: Find \(x\) such that \(f'(x) = 0\).
when \(x=2\) or when \(x=3\).
Step 3: Find \(x\) such that \(f'(x)\) does not exist.
Since \(f'(x)\) is polynomial, it exists for all real numbers.
Step 4: Verify that the values found in Steps 2 and 3 are in the domain of \(f\).
The domain of \(f(x)\) is all real numbers. Therefore, since both values are in the domain of \(f\), \(x=2\) and \(x=3\) are critical points of \(f\).
Example 2#
Find all critical points of \(f(x)=\dfrac{1}{x^2-1}\).
Step 1: Compute \(f'(x)\).
Step 2: Find \(x\) such that \(f'(x) = 0\).
\(f'(x) = 0\) when \(2x = 0\), which occurs when \(x=0\).
Step 3: Find \(x\) such that \(f'(x)\) does not exist.
\(f'(x)\) does not exist when \((x^2-1)^2 = 0\), which occurs when \(x=1\) and when \(x=-1\).
Step 4: Verify that the values found in Steps 2 and 3 are in the domain of \(f\).
The domain of \(f(x)\) is all real numbers except \(x=1\) and \(x=-1\). Therefore, \(x=0\) is a critical point, but \(x=1\) and \(x=-1\) are not critical points since they are not in the domain of \(f\).