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Calculus Level 1

Find the value of the limit lim x 0 e x 1 x . \lim_{x\to 0}\frac{e^x-1}{x}.


The answer is 1.

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5 solutions

lim x 0 e x 1 x = lim x 0 e i ( i x ) 1 x \lim_{x\to 0} \frac{e^x-1}{x}=\lim_{x\to 0} \frac{e^{i(-ix)} -1}{x} = lim x 0 cos ( i x ) i sin ( i x ) 1 x = lim x 0 cos ( i x ) 1 x i lim x 0 sin ( i x ) x =\lim_{x\to 0} \frac{\cos(ix)-i\sin(ix)-1}{x}=\lim_{x\to 0}\frac{\cos(ix)-1}{x}-i\lim_{x\to 0}\frac{\sin(ix)}{x} = lim x 0 sin ( i x ) sin ( i x ) x ( cos ( i x ) + 1 ) i 2 lim x 0 sin ( i x ) i x = i lim x 0 sin ( i x ) sin ( i x ) i x ( cos ( i x ) + 1 ) + 1 1 =\lim_{x\to 0}\sin(ix)\cdot \frac{\sin(ix)}{x(\cos(ix)+1)}-i^2\lim_{x\to 0}\frac{\sin(ix)}{ix}=i\lim_{x\to 0}\sin(ix)\cdot \frac{\sin(ix)}{ix(\cos(ix)+1)}+1\cdot 1 i 0 + 1 = 1 i\cdot 0 +1=1

Nice way to derive the limit through first principles :)

In a similar manner, we can show that ( e x ) = e x (e^x)' = e^x .

Calvin Lin Staff - 4 years, 7 months ago

why you deleted ur solution , sir ? @Chew-Seong Cheong

A Former Brilliant Member - 4 years, 7 months ago

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It is not the solution required.

Chew-Seong Cheong - 4 years, 7 months ago
Khor Ec
Apr 27, 2017

let f(x)= e x e^x , so

lim x 0 e x 1 x = lim x 0 f ( x ) f ( 0 ) x 0 \lim_{x \rightarrow0}\frac{e^x-1}{x}=\lim_{x \rightarrow0}\frac{f(x)-f(0)}{x-0}

Observe that the limit above is the definition of derivative of f(x) at x=0, f'(x)= e x e^x f'(0)=1

so, lim x 0 e x 1 x \lim_{x \rightarrow0}\frac{e^x-1}{x} =1

nice solution, Khor! .)

Hjalmar Orellana Soto - 4 years, 1 month ago

Yeah but you are using the derivative of e^x as a fact while, in my opinion, the point is to prove it.

maximos stratis - 4 years, 1 month ago

You are using what you want to prove 😅

Misael Cureño - 2 years, 1 month ago
Siddharth Gour
Oct 13, 2017

By using Taylor series expansion of e^x.

For using Taylor series you have to know how to find the derivative of e x e^x without using the definition, can you?

Hjalmar Orellana Soto - 3 years, 8 months ago
Aashay Agarwal
Jun 15, 2021

Well A common problem for JEE students And almost everyone does by Taylor series expansion ..

Nikhil Kumar
Oct 18, 2017

Use the expansion of e^x by taylor

For using Taylor series you have to know how to find the derivative of e x e^x without using the definition, can you?

Hjalmar Orellana Soto - 3 years, 8 months ago

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