\sqrt{ } s Can Ruin Your Lives!

Calculus Level 3

If f ( 1 ) = 1 f(1)=1 and f ( 1 ) = 2 f'(1)=2 , find the closed form of lim x 1 f ( x ) 1 x 1 \displaystyle \lim_{x \to 1} \frac{\sqrt{f(x)} - 1}{\sqrt{x} - 1} .

Give your answer to 3 decimal places.

Bonus : Evaluate this limit without applying L'Hôpital's Rule .


The answer is 2.000.

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2 solutions

lim x 1 f ( x ) 1 x 1 \large \displaystyle \lim_{x \rightarrow 1} \frac{\sqrt{f(x)} - 1}{\sqrt{x} - 1}

Multiplying top and bottom by ( x + 1 ) ( f ( x ) + 1 ) (\sqrt{x}+1)(\sqrt{f(x)}+1) ,

lim x 1 0 f ( x ) 1 x 1 × lim x 1 ( x ) + 1 f ( x ) + 1 \large \displaystyle \lim_{x-1 \rightarrow 0} \frac{f\left(x\right)-1}{x-1} \times \lim_{x \rightarrow 1} \frac{\sqrt{\left(x\right)}+1}{\sqrt{f\left(x\right)}+1}

lim x 1 0 f ( x ) 1 x 1 × ( 1 ) + 1 f ( 1 ) + 1 \large \displaystyle \lim_{x-1 \rightarrow 0} \frac{f\left(x\right)-1}{x-1} \times \frac{\sqrt{\left(1\right)}+1}{\sqrt{f\left(1\right)}+1}

Let x 1 = h x-1=h , then:

lim h 0 f ( 1 + h ) f ( 1 ) h × 1 + 1 1 + 1 \large \displaystyle \lim_{h \rightarrow 0} \frac{f\left(1+h\right)-f\left(1\right)}{h} \times \frac{1+1}{1+1}

f ( 1 ) × 1 = 2 f'(1) \times 1 = \boxed{2}

Gautam Sachdeva
Jun 6, 2017

Why it is written (give your answer in 3 decimal places??😂)

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