Inflection Point
An inflection point is a point where the graph of the function changes CONCAVITY (from up to down or vice versa).
Example
Draw a tangent line in imagination and move it on the function from left to right
Notice the tangent line's slope, does it go faster or slower or suddenly change its pace at a point?
We found it suddenly changed at point
c
.
More definitional way to solve:
Looking for the
parts of concavity shapes
Seems that
B-C
is a part ofConcave Down
, andC-D
is a part ofConcave Up
So
C
is a SWITCHING POINT, it's ainflection point
.
Example
Looking for the
parts of concavity shapes
There's no changing of concavity shapes, there's only one shape: Concave down.
Example
Notice that's the graph of
f'(x)
, which is the First Derivative.Checking
Inflection point
from 1st Derivative is easy: just to look at the change of direction.Obviously there're only two points changed direction: -1 & 2
Example
Mind that this is the graph of
f''(x)
, which is the Second derivative.Checking
inflection points
from 2nd derivative is even easier: just to look at when it changes its sign, or say crosses the X-axis.Obviously, it crosses the X-axis 5 times. So there're 5 inflection points of
f(x)
.
Example: Finding Inflection points
Function has POSSIBLE inflection points when
f''(x) = 0
.Set
f''(x) =0
and solve forx
, gotx=-3
.We now know the possible point, but don't know its CONCAVITY. This need to try some numbers from its both sides:
So it didn't change the concavity at point
-3
, means there's no inflection point for function.
Example: Finding Inflection points
Function has POSSIBLE inflection points when
f''(x) = 0
.Set
f''(x) =0
and solve forx
, gotx=0 or 6
.[Refer to Symbolab for
f''(x)
.](https://www.symbolab.com/solver/step-by-step/\frac{d^{2}}{dx^{2}}\left(3x^{5}-30x^{4}\right))We now know the possible point, but don't know its CONCAVITY. This need to try some numbers from its both sides:
So it didn't change the concavity at point
0
, means only6
is the inflection point.
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