find the lim #1

Calculus Level 2

lim x 1 ln ( x 2 + x 1 ) x 2 1 = ? \lim_{x \to 1} \frac {\ln(x^2+x-1)}{x^2-1} =\ ?


The answer is 1.5.

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2 solutions

Chew-Seong Cheong
Jun 29, 2020

L = lim x 1 ln ( x 2 + x 1 x 2 1 A 0/0 case, L’H o ˆ pital’s rule applies. = lim x 1 2 x + 1 x 2 + x 1 2 x Differentiate up and down w.r.t. x = 3 2 = 1.5 \begin{aligned} L & = \lim_{x \to 1} \frac {\ln(x^2+x-1}{x^2-1} & \small \blue{\text{A 0/0 case, L'Hôpital's rule applies.}} \\ & = \lim_{x \to 1} \frac {\frac {2x+1}{x^2+x-1}}{2x} & \small \blue{\text{Differentiate up and down w.r.t. }x} \\ & = \frac 32 = \boxed{1.5} \end{aligned}

Let x 1 = y x-1=y . Then the given expression equals ln ( y 2 + 3 y + 1 ) y 2 + 2 y \dfrac {\ln (y^2+3y+1)}{y^2+2y} .

When x 1 , y 0 x\to 1, y\to 0 , and the expression assumes the form 0 0 \dfrac {0}{0} .

So, applying L'Hospital's rule, we get the limit as 2 × 0 + 3 ( 0 2 + 3 × 0 + 1 ) ( 2 × 0 + 2 ) \dfrac {2\times 0+3}{(0^2+3\times 0+1)(2\times 0+2)}

= 3 2 = 1.5 =\dfrac {3}{2}=\boxed {1.5} .

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