For Julian I

Calculus Level 1

lim h 0 e 2 h 1 h = ? \large \lim_{h \rightarrow 0} \frac{e^{2h}-1}{h} = \ ?

Bonus: Don't use the hospital.


The answer is 2.00.

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6 solutions

Chew-Seong Cheong
Jun 13, 2015

We can use Maclaurin series as follows:

lim h 0 e 2 h 1 h = lim h 0 ( 1 + 2 h + ( 2 h ) 2 2 ! + ( 2 h ) 3 3 ! + . . . ) 1 h = lim h 0 ( 2 + 2 2 h 2 ! + 2 3 h 2 3 ! + 2 4 h 3 4 ! + . . . ) = 2 \begin{aligned} \lim_{h \to 0} {\frac{e^{2h}-1}{h}} & = \lim_{h \to 0} {\frac{\left(1 + 2h + \frac{(2h)^2}{2!} + \frac{(2h)^3}{3!} + ... \right) -1}{h}} \\ & = \lim_{h \to 0} \left( 2 + \frac{2^2h}{2!} + \frac{2^3h^2}{3!} + \frac{2^4h^3}{4!} + ... \right) \\ & = \boxed{2} \end{aligned}

Moderator note:

Can you think of a simpler approach? Hint: What is the definition of a derivative? Define f ( x ) = e 2 x f(x) = e^{2x} . What does e 2 x e 2 0 x 0 \large \frac{e^{2x} - e^{2\cdot 0} }{x - 0} mean? Slope is the ratio of rise and run.

Thanks. Ishan has shown it.

Chew-Seong Cheong - 6 years ago

I did it the same way. e^(2h) is just 1+2h plus a bunch of higher order terms, so clearly (1+2h-1)/h=2

Nathanael Case - 5 years, 11 months ago

By the difference of two squares, e 2 h 1 = ( e h + 1 ) ( e h 1 ) e^{2h}-1=(e^h+1)(e^h-1) , so lim h 0 e 2 h 1 h = 2 × lim h 0 e h 1 h \displaystyle\lim_{h\to0} \dfrac{e^{2h}-1}{h}=2\times\lim_{h\to0}\dfrac{e^h-1}{h} , because e 0 + 1 = 2 e^0+1=2 . But, remember your calculus 1 classes? The derivative of a exponential function is, by definition, d d x n x = lim h 0 n x + h n x h = n x × lim h 0 n h 1 h \dfrac{d}{dx}n^x=\displaystyle\lim_{h\to0}\dfrac{n^{x+h}-n^x}{h}=n^x\times\lim_{h\to0}\dfrac{n^h-1}{h} , and, by definition, e e is the number which lim h 0 n h 1 h = 1 \displaystyle\lim_{h\to0}\dfrac{n^h-1}{h}=1 , in order to find the exponencial function which is its own derivative. So, 2 × lim h 0 e h 1 h = 2 × 1 = 2 2\times\displaystyle\lim_{h\to0}\dfrac{e^h-1}{h}=2\times1=\boxed{2} .

Sai Raghava Puni
Jun 18, 2015

Let 2h=t,as h->0 t->0 then it becomes 2lim e^t -1/t = 2 as t->0

Cheat, use a sci calculator and set h = to a small value, say .001, and calculate directly.

It is known that lim h 0 e h 1 h = 1 \displaystyle\lim_{h\rightarrow0}\dfrac{e^h-1}{h}=1 (don't trust me? Check a proof for e x e^x being its own derivative). Now, lim h 0 e 2 h 1 h = lim h 0 e h 1 h ( e h + 1 ) = lim h 0 e h 1 h lim h 0 ( e h + 1 ) = 1 ( 1 + 1 ) = 2 \displaystyle\lim_{h\rightarrow0}\dfrac{e^{2h}-1}{h}=\lim_{h\rightarrow0}\dfrac{e^h-1}{h}\cdot(e^h+1)=\lim_{h\rightarrow0}\dfrac{e^h-1}{h}\cdot\lim_{h\rightarrow0}(e^h+1)=1\cdot(1+1)=2 .

Moderator note:

Assuming the fact that you used, there is a much more direct approach that allows you to generalize to all real values r r . What is

lim h \righarrow 0 e r h 1 h ? \lim_{ h \righarrow 0 } \frac{ e ^ { rh } - 1 } { h }?

P.S. Actually, for the fact that you stated, the limit is just e h 1 h \frac{ e^h - 1 } { h } . Be careful what you are taking limits with respect to :)

Thanks! Already corrected that. :)

José Marín Guzmán - 5 years, 11 months ago
Adrian Peasey
Jun 15, 2015

The Taylor expansion of e x e^x about x = 0 x=0 is

n = 0 x n n ! \displaystyle \sum_{n=0}^\infty \frac{x^n}{n!}

The first two terms of the expansion of e 2 h e^{2h} are 1 + 2 h 1+2h which gives 1 + 2 h 1 h = 2 h h = 2 \frac{1+2h-1}{h}=\frac{2h}{h}=\boxed{2}

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