Try the fastest way to create flashcards
Question

Use the alternative form of the derivative to find the derivative at x=c (if it exists).f(x) = 3/x, c=4

Solution

Verified
Answered 9 months ago
Answered 9 months ago
Step 1
1 of 2

To solve this question, we use the alternative form of the derivative.

f(x)=limxcf(x)f(c)xc\begin{align*} f'(x)=\lim_{x\rightarrow c}{\dfrac{f(x)-f(c)}{x-c}} \end{align*}

Using this we have

f(4)=limx4f(x)f(4)x4=limx43x34x41\begin{align*} f'(4)&=\lim_{x\rightarrow 4}{\dfrac{f(x)-f(4)}{x-4}}\\\\ &=\lim_{x\rightarrow 4}{\dfrac{\dfrac{3}{x}-\dfrac{3}{4}}{\dfrac{x-4}{1}}}\tag{1} \end{align*}

Because fractions in numerator have different denominators the first step is to find equivalent fractions which have common denominator. We find the Least Common Denominator (LCD) by multiplying denominators: 4x4x then rewrite fractions in the equation as equivalent fractions using the LCD as denominator. Then add and simplify.

(1)=limx4433x4xx41=limx43(x4)4xx41=limx43(x4)4x(x4)=limx434x=344=316\begin{align*} (1)&=\lim_{x\rightarrow 4}{\dfrac{\dfrac{4\cdot3-3x}{4x}}{\dfrac{x-4}{1}}} \\\\ &=\lim_{x\rightarrow 4}{\dfrac{\dfrac{-3(x-4)}{4x}}{\dfrac{x-4}{1}}}\tag{Factor out -3.}\\\\ &=\lim_{x\rightarrow 4}{\dfrac{-3\cancel{(x-4)}}{4x\cancel{(x-4)}}}\tag{Simplify complex fraction.}\\\\ &=\lim_{x\rightarrow 4}{\dfrac{-3}{4x}}\\\\ &=\dfrac{-3}{4\cdot 4}\\\\ &={\color{Mahogany}\dfrac{-3}{16}} \end{align*}

Create a free account to view solutions

Create a free account to view solutions

Recommended textbook solutions

Thomas' Calculus 14th Edition by Christopher E Heil, Joel R. Hass, Maurice D. Weir

Thomas' Calculus

14th EditionISBN: 9780134438986 (11 more)Christopher E Heil, Joel R. Hass, Maurice D. Weir
10,142 solutions
Calculus 10th Edition by Bruce H. Edwards, Ron Larson

Calculus

10th EditionISBN: 9781285057095 (3 more)Bruce H. Edwards, Ron Larson
12,387 solutions
Calculus: Early Transcendentals 8th Edition by James Stewart

Calculus: Early Transcendentals

8th EditionISBN: 9781285741550 (5 more)James Stewart
11,084 solutions
Calculus: Early Transcendentals 9th Edition by Daniel K. Clegg, James Stewart, Saleem Watson

Calculus: Early Transcendentals

9th EditionISBN: 9781337613927Daniel K. Clegg, James Stewart, Saleem Watson
11,050 solutions

More related questions

1/4

1/7