#23 Measure Your Calibre

Calculus Level 3

If f ( x ) = x 1 + x 3 \large{f(x) = |x-1| + |x-3|}

Find f ( x ) \large{f'(x)} at x = 4 x=4 .


Check your Calibre


The answer is 2.

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3 solutions

Viki Zeta
Mar 14, 2017

d d x x = x x f ( x ) = x 1 + x 3 f ( x ) = x x + x x f ( 4 ) = 4 4 + 4 4 = 1 + 1 = 2 \dfrac{\mathrm{d}}{\mathrm{d}x} |x| = \dfrac{|x|}{x} \\ f(x) = |x-1| + |x-3| \\ f'(x) = \dfrac{|x|}{x} + \dfrac{|x|}{x} \\ f'(4) = \dfrac{|4|}{4} + \dfrac{|4|}{4} \\ = 1 + 1 = 2

Your third step is incorrect , it should be

f ( x ) = x 1 x 1 + x 3 x 3 f^{'}(x) = \dfrac{x-1}{|x-1|}+\dfrac{x-3}{|x-3|}

Sabhrant Sachan - 4 years, 2 months ago

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Whats the big difference between that and what i wrote? It's all same.

Viki Zeta - 4 years, 2 months ago

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No your answer came correct but it will be 4-1/4-1 + 4-3/4-3 =2. Yiur answer is correct but you have dine it wron brother

Md Zuhair - 4 years, 2 months ago

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@Md Zuhair It's actually correct. View wiki of modulus or absolute derivative

Viki Zeta - 4 years, 2 months ago

What is the proof that d x d x \dfrac{d|x|}{dx} = x x \dfrac{|x|}{x} ?

Achal Jain - 4 years, 2 months ago

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As |x|=y . By squaring and differentuating we get dy/dx = x x \frac{|x|}{x}

Md Zuhair - 4 years, 2 months ago

check it out

Sabhrant Sachan - 4 years, 2 months ago

Wrong. It will be x 3 x 3 \dfrac {|x-3|}{x-3} will be the 1st expressions derivative

Md Zuhair - 4 years, 2 months ago

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The derivative of an absolute or a modulus gives the absolute integer form of the number, ie +1 or -1

Viki Zeta - 4 years, 2 months ago
Nivedit Jain
Mar 14, 2017

Let's consider x is greater than 3 so equation becomes y=2x -3 simply slope is 2

x > 3 f ( x ) = 2 x 4 f ( 4 ) = 2 \forall x>3 f(x)=2x-4 \Rightarrow f'(4)=2

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