Find the degree in the limit

Calculus Level 3

If lim x 1 x n + 2 x 3 x 1 = 25 \displaystyle \lim_{x \to 1} \frac{x^n + 2x - 3}{x-1} = 25 , what is the value of n n ?


The answer is 23.

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8 solutions

Josh Petrin
Dec 14, 2013

Notice that ( 1 ) n + 2 ( 1 ) 3 1 1 = 0 0 \frac{(1)^n + 2(1) - 3}{1 - 1} = \frac{0}{0} . So we can simplify the top and bottom expression by L'Hopital's Rule; we differentiate both the numerator and the denominator. lim x 1 x n + 2 x 3 x 1 = lim x 1 n x n 1 + 2 1 = 2 + lim x 1 n x n 1 = 2 + n ( 1 ) n 1 = 2 + n \begin{aligned} \lim_{x \to 1} \frac{x^n + 2x - 3}{x - 1} &= \lim_{x \to 1} \frac{nx^{n-1} + 2}{1} \\ &= 2 + \lim_{x \to 1} nx^{n-1} \\ &= 2 + n(1)^{n-1} \\ &= 2 + n \end{aligned} Thus, n + 2 = 25 n + 2 = 25 , and n = 23 n = \boxed{23} .

what happen if i told you that, you don't need use H'lopital rules for find the answer x n 1 x 1 + 2 x 2 x 1 = x 1 + x 2 + x 3 + . . . + x n 1 + 2 = 25 \frac{x^n-1}{x-1}+\frac{2x-2}{x-1}=x^1+x^2+x^3+...+x^{n-1}+2=25

Pablo Cesar Herrera Ortiz - 7 years, 5 months ago

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Nice! I didn't see that, but that's a great solution!

Josh Petrin - 7 years, 5 months ago

I did the same thing. You are just finding the value of n that makes this statement true where x = 1 x=1 . 2 + i = 1 n x i = 25 2+\sum_{i=1}^n x^i=25

Trevor B. - 7 years, 5 months ago

You should add another 1 1 since x n 1 x 1 = 1 + x + x n 1 \frac{x^n-1}{x-1}= 1+ x+ \cdots x^{n-1} .

A Brilliant Member - 7 years, 5 months ago

Ahh... I see my mistake, forgot the 2...

Justin Chan - 7 years, 3 months ago
Parth Kohli
Dec 14, 2013

Differentiating the numerator and denominator (L'Hôpital's Rule), lim x 1 x n + 2 x 3 x 1 = lim x 1 n x n 1 + 2 1 \lim_{x \to 1} \dfrac{x^n + 2x - 3}{x - 1} = \lim_{x \to 1}\dfrac{nx^{n-1} + 2}{1} Now, it is convenient to plug-in 1 1 to the limit. We are left with the following: n 1 ( n 1 ) + 2 = 25 n + 2 = 25 n = 23 n\cdot 1^{(n-1)} + 2 = 25 \Rightarrow n + 2 = 25 \Rightarrow n = \boxed{23}

Sai Arvind
Dec 14, 2013

*using l hospital method * and putting x=1 we get nx1+2=25 so n=23

Rik Ghosh
Dec 13, 2013

by L' Hospitals rule we differentiate both numerator and denominator to get nx^(n-1)+2 and put x=1 to get n = 23

Kishore Himanshu
Feb 28, 2014

Use L'Hopital Rule. Differentiate both the numerator and denominator w.r.t. x

Harshil Patel
Jan 1, 2014

Let start this problem using L'Hôpital's rule

L'Hôpital's rule states that

lim x c f ( x ) \lim_{x \to c} f(x) = lim x c g ( x ) \lim_{x \to c} g(x) = 0 or ± \pm \infty and,

lim x c \lim_{x \to c} f ( x ) g ( x ) \frac{f'(x)}{g'(x)} exists and g ( x ) g'(x) \not= 0 for all x,

then lim x c \lim_{x \to c} f ( x ) g ( x ) \frac{f(x)}{g(x)} = lim x c \lim_{x \to c} f ( x ) g ( x ) \frac{f'(x)}{g'(x)}

So let Differentiate Numerator and Denominator individually,

So we are left with lim x 1 \lim_{x \to 1} n x n 1 + 2 = 25 nx^{n-1} + 2 = 25

Replacing x = 1 x = 1 we get n + 2 = 25 n = 23 n + 2 = 25 \Longrightarrow n = \boxed{23}

THE MOST ELEGANT SOLUTION... GREAT WORK EULER!!!!

kirtan bhatt - 7 years, 5 months ago
Budi Utomo
Dec 23, 2013

With L'Hospital ---> become {n.x^(n-1) + 2 }/ 1 = 25 ----> n*1 + 2 = 25 ---> n = 25 - 2 = 23. Answer : 23

Michael Thornton
Dec 22, 2013

If we replace x with 1 for every term involving x, we get an expression which looks like this: 1 n + 2 × 1 3 1 1 = 25 \frac{1^n + 2 \times 1 - 3}{1 - 1} = 25

This leaves us with 0 in the denominator which doesn't work, so we must use L'Hopital's rule to answer this question. L'hopital's rule basically says that if we replace the expression in the numerator ( x n + 2 x 3 x^n + 2x - 3 ) with it's derivative function ( n x n 1 + 2 nx^{n-1} + 2 ), and the expression in the denominator ( x 1 x-1 ) with it's own derivative function ( 1 1 ), we will render the same limit value as we would had we evaluated the original expressions. Therefore we end up with this:

n x n 1 + 2 1 = 25 \frac{nx^{n-1} + 2}{1} = 25

which - as there is 1 in the denominator - can be written as:

n x n 1 + 2 = 25 nx^{n^-1} + 2 = 25

From here we can subtract two from both sides:

n x n 1 = 23 nx^{n-1} = 23

Now we can solve n. We have been told that as x approaches 1 the limit tends to 23, so we can go straight ahead and see whether substituting 1 into x works. It does - 1 to the power of any natural number such as n n is always equal to 1, and therefore n × 1 n 1 = 23 n \times 1^{n-1} = 23 will easily render a value for n. All we need to do now is divide 23 by one (rearrange to 23 x n 1 = n \frac{23}{x^{n-1}} = n ) leaving us with the answer:

n = 23 n = \boxed{23}

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