Derivatives of Products and Quotients

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Barnett/Ziegler/Byleen Business Calculus 11e Derivatives of Products In words: The derivative

Barnett/Ziegler/Byleen Business Calculus 11e

Derivatives of Products

In words: The derivative of the

product of two functions is the first function times the derivative of the second function plus the second function times the derivative of the first function.

Theorem 1 (Product Rule)
If f (x) = F(x) ⋅ S(x), and if F ’(x) and S ’(x) exist, then
f ’ (x) = F(x) ⋅ S ’(x) + F ’(x) ⋅ S(x), or

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Barnett/Ziegler/Byleen Business Calculus 11e Example Find the derivative of y = 5x2(x3 + 2).

Barnett/Ziegler/Byleen Business Calculus 11e

Example

Find the derivative of y = 5x2(x3 +

2).
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Barnett/Ziegler/Byleen Business Calculus 11e Example Find the derivative of y =

Barnett/Ziegler/Byleen Business Calculus 11e

Example

Find the derivative of y = 5x2(x3 +

2).
Solution:
Let F(x) = 5x2, so F ’(x) = 10x Let S(x) = x3 + 2, so S ’(x) = 3x2.
Then
f ’ (x) = F(x) ⋅ S ’(x) + F ’(x) ⋅ S(x)
= 5x2 ⋅ 3x2 + 10x ⋅ (x3 + 2)
= 15x4 + 10x4 + 20x = 25x4 + 20x.
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Barnett/Ziegler/Byleen Business Calculus 11e Theorem 2 (Quotient Rule) If f (x)

Barnett/Ziegler/Byleen Business Calculus 11e

Theorem 2 (Quotient Rule)
If f (x) = T

(x) / B(x), and if T ’(x) and B ’(x) exist, then
or

Derivatives of Quotients

In words: The derivative of the quotient of two functions is the bottom function times the derivative of the top function minus the top function times the derivative of the bottom function, all over the bottom function squared.

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Barnett/Ziegler/Byleen Business Calculus 11e Example Find the derivative of y = 3x / (2x + 5).

Barnett/Ziegler/Byleen Business Calculus 11e

Example

Find the derivative of y = 3x /

(2x + 5).
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Barnett/Ziegler/Byleen Business Calculus 11e Example Find the derivative of y =

Barnett/Ziegler/Byleen Business Calculus 11e

Example

Find the derivative of y = 3x /

(2x + 5).
Solution:
Let T(x) = 3x, so T ’(x) = 3 Let B(x) = 2x + 5, so B ’(x) = 2.
Then
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Barnett/Ziegler/Byleen Business Calculus 11e Tangent Lines Let f (x) = (2x

Barnett/Ziegler/Byleen Business Calculus 11e

Tangent Lines

Let f (x) = (2x - 9)(x2

+ 6). Find the equation of the line tangent to the graph of f (x) at x = 3.
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Barnett/Ziegler/Byleen Business Calculus 11e Tangent Lines Let f (x) = (2x

Barnett/Ziegler/Byleen Business Calculus 11e

Tangent Lines

Let f (x) = (2x - 9)(x2

+ 6). Find the equation of the line tangent to the graph of f (x) at x = 3.
Solution: First, find f ’(x):
f ’(x) = (2x - 9) (2x) + (2) (x2 + 6)
Then find f (3) and f ’(3):
f (3) = -45 f ’(3) = 12
The tangent has slope 12 and goes through the point (3, -45).
Using the point-slope form y - y1 = m(x - x1), we get
y – (-45) = 12(x - 3) or y = 12x - 81