Does this limit exist?

Calculus Level 3

lim x 0 ( 1 + x ) x 1 x 2 = ? \large \lim_{x\to 0}\dfrac{(1+x)^x-1}{x^2}= \, ?


The answer is 1.

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3 solutions

Gabriel Merces
Mar 29, 2014

The Limit is in the Indeterminate Form 0 0 \frac{0}{0}

Applying L'Hopital's Rule Yields :

lim x 0 ( 1 + x ) x 1 x 2 \lim_{x\to 0}\frac{(1+x)^x-1}{x^2}

= [ ( 1 + x ) x ] 2 x \frac{[(1+x)^x]'}{2x}

= lim x 0 ( 1 + x ) x ln ( 1 + x ) + x ( 1 + x ) x 1 2 x \lim_{x\to 0}\frac{(1+x)^x\ln (1+x)+x(1+x)^{x-1}}{2x}

= lim x 0 ( 1 + x ) x 1 2 ln ( 1 + x ) x + lim x 0 ( 1 + x ) x 1 2 \lim_{x\to 0}(1+x)^x\cdot\frac{1}{2}\cdot\frac{\ln (1+x)}{x}+\lim_{x\to 0}\frac{(1+x)^{x-1}}{2}

= 1 1 2 1 + 1 2 = 1 1\cdot\frac{1}{2}\cdot 1+\frac{1}{2}=1

Thus Answer Is 1 \boxed{1}

\cdots \cdots \cdots \cdots \cdots

Note That

[ ( 1 + x ) x ] = [ e x ln ( 1 + x ) ] [(1+x)^x]' = [e^{x\ln (1+x)}]'

= e x ln ( 1 + x ) [ x ln ( 1 + x ) ] e^{x\ln (1+x)}\cdot [x\ln (1+x)]'

= ( 1 + x ) x ( ln ( 1 + x ) + x x + 1 ) (1+x)^x\left(\ln (1+x)+\frac{x}{x+1}\right)

= ( 1 + x ) x ln ( 1 + x ) + ( 1 + x ) x 1 x (1+x)^x\ln (1+x)+(1+x)^{x-1}x

Better way:

( 1 + x ) x = e x ln ( 1 + x ) (1+x)^x = e^{x \ln(1+x)}

lim x 0 ( 1 + x ) x 1 x 2 \displaystyle \lim_{x \to 0} \frac{(1+x)^x - 1}{x^2}

= lim x 0 e x ln ( 1 + x ) 1 x 2 \displaystyle \lim_{x \to 0} \frac{e^{x \ln(1+x)} - 1}{x^2}

= lim x 0 x ln ( 1 + x ) x 2 = lim x 0 x 2 x 2 = 1 \displaystyle \lim_{x \to 0} \frac{x \ln(1+x)}{x^2} = \lim_{x \to 0} \frac{x^2}{x^2} = 1

Using lim t 0 e t 1 = t , lim t 0 ln ( 1 + t ) = t \displaystyle \lim_{t \to 0} e^t - 1 = t, \lim_{t \to 0} \ln(1+t) = t

jatin yadav - 7 years, 2 months ago

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Possibly a faster way is the following:

lim x 0 ( 1 + x ) x 1 x 2 \lim_{x\to 0} \frac{(1+x)^x-1}{x^2}

As x 0 x\to 0 , we have that ( 1 + x ) n = 1 + n x (1+x)^n = 1+nx

lim x 0 1 + ( x ) x 1 x 2 \lim_{x\to 0} \frac{1+(x)x-1}{x^2}

lim x 0 x 2 x 2 = 1 \lim_{x\to 0} \frac{x^2}{x^2} = \boxed{1}

Anish Puthuraya - 7 years, 2 months ago

Good solution bro.

Hisham Sacar - 7 years, 2 months ago
Otto Bretscher
Mar 13, 2016

( 1 + x ) x 1 = e ln ( 1 + x ) x 1 = e x 2 + o ( x 2 ) 1 = x 2 + o ( x 2 ) (1+x)^x-1=e^{\ln(1+x)x}-1=e^{x^2+o(x^2)}-1=x^2+o(x^2) so that the limit is 1 \boxed{1}

We are using the Taylor series ln ( 1 + x ) = x + o ( x ) \ln(1+x)=x+o(x) and e x = 1 + x + o ( x ) e^x=1+x+o(x)

Here we say that a function f ( x ) f(x) is o ( x k ) o(x^k) if lim x 0 f ( x ) x k = 0 \lim_{x\to 0}\frac{f(x)}{x^k}=0

Ritwik Shukla
Apr 1, 2014

(1+x)n =1+nx if x is very small ∴ (1+x)x =1+x2 ∴lim x->0 ((1+x)x -1)/x2 =x2/x2=1

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