[College Calc 01-03. Limits of Functions] #01

Calculus Level 3

Given

lim x 0 ( x 2 e x x + 6 x e x x ) = p + ln q \lim_{x\to 0}\left(\frac{x^2}{e^x-x}+\frac{6^x-e^x}{x}\right)=p+\ln q

for rationals p , q , p, q, find the value of p + q . p+q.


This is a part of the College Calc problem set. You can find more problems here .

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2 solutions

Boi (보이)
Jan 9, 2021

Note that

lim x 0 x 2 e x x = 0 1 0 = 0 ; \lim_{x\to 0}\frac{x^2}{e^x-x} = \frac{0}{1 - 0} = 0;

be careful not to use l'hopital on the limit above. As for the second term,

lim x 0 6 x e x x = lim x 0 ( 6 x 1 ) ( e x 1 ) x = ln 6 ln e = 1 + ln 6. \lim_{x\to 0}\frac{6^x-e^x}{x} = \lim_{x\to 0}\frac{(6^x-1)-(e^x-1)}{x}=\ln 6 - \ln e = -1 + \ln 6.

Therefore, p = 1 p=-1 and q = 6 q=6 such that p + q = 5. p+q = 5.

ChengYiin Ong
Jan 18, 2021

We can use L'hopital when we combine the fractions! lim x ( x 2 e x x + 6 x e x x ) = lim x 0 x 3 + 6 x e x e 2 x x 6 x + x e x x e x x 2 = L’H 0 0 lim x 0 3 x 2 + ln ( 6 e ) 6 x e x 2 e 2 x 6 x ln 6 x 6 x + e x + x e x e x + x e x 2 x = ln 6 + 1 2 1 + 1 = ln 6 1 \lim_{x\rightarrow} \left(\frac{x^2}{e^x-x}+\frac{6^x-e^x}{x}\right)=\lim_{x\rightarrow 0} \frac{x^3+6^xe^x-e^{2x}-x6^x+xe^x}{xe^x-x^2}\overset{\textrm{L'H} \frac{0}{0}}{=}\lim_{x\rightarrow 0} \frac{3x^2+\ln (6e)6^xe^x-2e^{2x}-6^x-\ln 6 x6^x+e^x+xe^x}{e^x+xe^x-2x}=\ln 6+1-2-1+1=\boxed{\ln 6-1}

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