Logarithms Limits

Calculus Level 2

lim x 0 ln ( x 3 + 3 x 2 + 3 x + 1 ) x 2 + 6 x = ? \lim_{x \to0} {\dfrac{\ln{(x^3 + 3x^2 + 3x + 1)}}{x^2 + 6x}} = \, ?


The answer is 0.5.

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3 solutions

Mehul Arora
Feb 29, 2016

lim x 0 ln ( x + 1 ) 3 x 2 + 6 x \lim_{x\to 0} \dfrac {\ln (x+1)^3}{x^2 +6x}

lim x 0 3 ln ( x + 1 ) x 2 + 6 x \lim_{x\to 0} \dfrac {3 \ln (x+1)}{x^2 +6x}

Using L'Hopitals rule :

lim x 0 3 × 1 ( x + 1 ) 2 x + 6 \lim_{x\to 0} \dfrac {3 \times \dfrac {1}{(x+1)}}{2x+6}

Now putting x = 0 x=0 , we get lim x 0 ln ( x 3 + 3 x 2 + 3 x + 1 ) x 2 + 6 x = 1 2 = 0.5 \lim_{x \rightarrow 0} {\dfrac{\ln{(x^3 + 3x^2 + 3x + 1)}}{x^2 + 6x}} = \dfrac {1}{2}= 0.5

Soumava Pal
Feb 29, 2016

We can write the given fraction as

l n ( ( x + 1 ) 3 ) x ( x + 6 ) \frac{ln((x+1)^3)}{x(x+6)}

= 3 x + 6 \frac{3}{x+6} * l n ( x + 1 ) x \frac{ln(x+1)}{x}

Taking limit at x going to 0, the second fraction goes to 1, according to the standard result on logarithmic limits , and the first fraction goes to 1 2 \frac{1}{2} .

First step for lim x 0 ( ln ( x 3 + 3 x 2 + 3 x + 1 ) x 2 + 6 x ) \lim_{x\to 0} \left(\frac{\ln(x^3+3x^2+3x+1)}{x^2+6x}\right)

Steps 1.Use L'Hopital's rule

2.Find d d x ( ln ( x 3 + 3 x 2 + 3 x + 1 ) ) \frac{d}{dx}(\ln(x^3+3x^2+3x+1)) and d d x ( x 2 + 6 x ) \frac{d}{dx}(x^2+6x)

3. 3 6 \frac{3}{6} reduce into 1 2 \frac{1}{2}

\square

FIN!!! \LARGE \text{FIN!!!}

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