Using L'Hospital's Rule?

Calculus Level 3

lim x 2 63 x 5 630 x 4 + 2520 x 3 5040 x 2 + 5040 x 2016 126 x 4 1008 x 3 + 3024 x 2 4032 x + 2016 = ? \large \lim_{x\to2} \dfrac{63x^5-630x^4+2520x^3-5040x^2+5040x-2016}{126x^4-1008x^3+3024x^2-4032x+2016} = \, ?


The answer is 0.

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3 solutions

Tommy Li
Jun 2, 2016

lim x 2 63 x 5 630 x 4 + 2520 x 3 5040 x 2 + 5040 x 2016 126 x 4 1008 x 3 + 3024 x 2 4032 x + 2016 \lim_{x\to2} \dfrac{63x^5-630x^4+2520x^3-5040x^2+5040x-2016}{126x^4-1008x^3+3024x^2-4032x+2016}

= lim x 2 63 ( x 5 10 x 4 + 40 x 3 80 x 2 + 80 x 32 ) 126 ( x 4 8 x 3 + 24 x 2 32 x + 16 ) = \lim_{x\to2} \dfrac{63(x^5-10x^4+40x^3-80x^2+80x-32)}{126(x^4-8x^3+24x^2-32x+16)}

= lim x 2 63 ( x 5 + 5 ( 2 ) x 4 + 10 ( 2 ) 2 x 3 + 10 ( 2 ) 3 x 2 + 5 ( 2 ) 4 x + ( 2 ) 5 ) 126 ( x 4 + 4 ( 2 ) x 3 + 6 ( 2 ) 2 x 2 + 4 ( 2 ) 3 x + ( 2 ) 4 ) = \lim_{x\to2} \dfrac{63(x^5+5(-2)x^4+10(-2)^2x^3+10(-2)^3x^2+5(-2)^4x+(-2)^5)}{126(x^4+4(-2)x^3+6(-2)^2x^2+4(-2)^3x+(-2)^4)}

= lim x 2 63 ( x 2 ) 5 126 ( x 2 ) 4 = \lim_{x\to2} \dfrac{63(x-2)^5}{126(x-2)^4} (Binomial Theorem)

= lim x 2 ( x 2 ) 2 = \lim_{x\to2} \dfrac{(x-2)}{2}

= 0 2 = \frac{0}{2}

= 0 = 0

L'Hospital's Rule is not involved. Ha!

Congratulations, :) (+1) Nice solution.

Ashish Menon - 5 years ago
Hung Woei Neoh
Jun 5, 2016

Using L'Hospital's Rule (I simply insist):

lim x 2 63 x 5 630 x 4 + 2520 x 3 5040 x 2 + 5040 x 2016 126 x 4 1008 x 3 + 3024 x 2 4032 x + 2016 [ 0 0 ] = L’H lim x 2 315 x 4 2520 x 3 + 7560 x 2 10080 x + 5040 504 x 3 3024 x 2 + 6048 x 4032 [ 0 0 ] = L’H lim x 2 1260 x 3 7560 x 2 + 15120 x 10080 1512 x 2 6048 x + 6048 [ 0 0 ] = L’H lim x 2 3780 x 2 15120 x + 15120 3024 x 6048 [ 0 0 ] = L’H lim x 2 7560 x 15120 3024 = 7560 ( 2 ) 15120 3024 = 15120 15120 3024 = 0 \displaystyle \lim_{x\to2} \dfrac{63x^5 -630x^4+2520x^3-5040x^2+5040x-2016}{126x^4-1008x^3+3024x^2-4032x+2016} \quad\quad\color{#3D99F6}{\left[\dfrac{0}{0}\right]}\\ \stackrel{\text{L'H}}=\displaystyle \lim_{x\to2} \dfrac{315x^4 -2520x^3+7560x^2-10080x+5040}{504x^3-3024x^2+6048x-4032} \quad\quad\quad\;\color{#3D99F6}{\left[\dfrac{0}{0}\right]}\\ \stackrel{\text{L'H}}=\displaystyle \lim_{x\to2} \dfrac{1260x^3 -7560x^2+15120x-10080}{1512x^2-6048x+6048} \quad\quad\quad\quad\quad\quad\;\,\color{#3D99F6}{\left[\dfrac{0}{0}\right]}\\ \stackrel{\text{L'H}}=\displaystyle \lim_{x\to2} \dfrac{3780x^2 -15120x+15120}{3024x-6048} \quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\;\;\color{#3D99F6}{\left[\dfrac{0}{0}\right]}\\ \stackrel{\text{L'H}}=\displaystyle \lim_{x\to2} \dfrac{7560x -15120}{3024} \\ =\dfrac{7560(2) - 15120}{3024}\\ =\dfrac{15120 - 15120}{3024}\\ =\boxed{0}

Yes, L'Hospital's Rule can solve the problem. You wrote a good solution too ! Thanks you very much ! However, I used alternative method.

Tommy Li - 5 years ago

Haha neat and eccentric (+1)

Ashish Menon - 5 years ago
Carsten Meyer
Aug 25, 2019

Let the numerator be p ( x ) p(x) and the denominator be q ( x ) q(x) . Use Horner's Method to rewrite them as polynomials in ( x 2 ) (x-2) :

\(\displaystyle\begin{align} p(x):&& \begin{array}{rrrrrr} 63 & -630 & 2520 & -5040 & 5040 & -2016 \\ \fbox{2} & 126 & -1008 & 3024 & -4032 & 2016 \\ \hline 63 & -504 & 1512 & -2016 & 1008 & \boxed{0:=p_0} \\ \fbox{2} & 126 & -756 & 1512 & -1008 & \\ \hline 63 & -378 & 756 & -504 & \boxed{0=:p_1} & \\ \fbox{2} & 126 & -504 & 504 & & \\ \hline 63 & -252 & 252 & \boxed{0=:p_2} & & \\ \fbox{2} & 126 & -252 & & & \\ \hline 63 & -126 & \boxed{0=:p_3} & & & \\ \fbox{2} & 126 & & & & \\ \hline \boxed{63=:p_5} & \boxed{0=:p_4} & & & & \end{array},&& p(x)=\sum_{k=0}^5p_k(x-2)^k=63(x-2)^5\\\\

q(x):&& \begin{array}{rrrrr} 126 & -1008 & 3024 & -4032 & 2016\\ \fbox{2} & 252 & -1512 & 3024 & -2016\\ \hline 126 & -756 & 1512 & -1008 & \boxed{0 =: q_0}\\ \fbox{2} & 252 & -1008 & 1008 & \\ \hline 126 & -504 & 504 & \boxed{0 =: q_1} &\\ \fbox{2} & 252 & -504 & & \\ \hline 126 & -252 & \boxed{0 =: q_2} & &\\ \fbox{2} & 252 & & & \\ \hline \boxed{126 =: p_4} & \boxed{0 =: q_3} & & & \end{array}, && q(x)=\sum_{k=0}^4 q_k(x-2)^k=126(x-2)^4 \end{align} \)

Putting all together: lim x 2 p ( x ) q ( x ) = lim x 2 63 ( x 2 ) 5 126 ( x 2 ) 4 = lim x 2 x 2 2 = 0 \displaystyle \lim_{x\rightarrow 2}\frac{p(x)}{q(x)}=\lim_{x\rightarrow 2}\frac{ 63(x-2)^{\cancel{5}} }{ 126\cancel{(x-2)^4} }=\lim_{x\rightarrow2}\frac{x-2}{2}=\boxed{0}

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