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5: Calculus
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Differentiation

Differentiation

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Analyzing function behavior

Stationary points & Increasing/Decreasing Regions

f′(x)⎩⎪⎨⎪⎧​<0⇔f decreasing=0⇔f stationary>0⇔f increasing​🚫


Example:

f(x)=x2+3x⇒f′(x)=2x+3


f′(x)=0whenx=−23​


Therefore, f(x) is:

  • Decreasing for x<−23​

  • Stationary for x=−23​

  • Increasing for x>−23​

Inflexion points

Inflexion points occur when f′′(x)=0 and f′′(x) changes sign. 🚫

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Example:

f(x)=x3+x⇒f′′(x)=6x
f′′(x)=0whenx=0


f′′(x)=6x changes sign from − to + at x=0, so there is an inflexion point at x=0.

Graphs of f, f' and f''

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  • When f′ crosses the x-axis f has a maximum or minimum

  • When f′′ crosses the x-axis, f has an inflexion point.

Maxima & Minima

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f′(x)=0 and f′′(x){<0→max>0→min​🚫


Example:

f(x)=x3−3x2​⇒f′(x)​ ⇒f′′(x)​=3x2−6x=3x(x−2)=6x−6​


So f′(x)=0 when x=0 or x=2.

f′′(0)=−6<0andf′′(2)=6>0
minimum at x=2
maximum at x=0

Concavity

f′′{>0⇔f concave up<0⇔f concave down​🚫

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