Oops! two limits

Calculus Level pending

lim x 1 lim y 0 ( e x y 2 ( ln ( ( x + y ) y ) e y + ln ( 1 x y ) ) ) = ? \large \lim_{x\to1} \lim_{y\to0} \left(\frac{e^x}{y^2}\left(\ln \left((x+y)^y\right)e^y+\ln\left(\frac{1}{x^y}\right) \right) \right) =?

1 e 0 1/e

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1 solution

Gilliën Fransz
Feb 5, 2018

note that: e x y 2 ( ln ( ( x + y ) y ) e y + ln ( 1 x y ) ) = e x y 2 ( y ln ( ( x + y ) ) e y y ln ( x ) ) = e x y y 2 ( ln ( x + y ) e y ln ( x ) ) \frac{e^x}{y^2}(\ln((x+y)^y)e^y+\ln(\frac{1}{x^y})) = \frac{e^x}{y^2}(y\ln((x+y))e^y-y\ln(x)) = \frac{e^x\color{#D61F06}y}{y^{\color{#D61F06}2}}(\ln(x+y)e^y-\ln(x)) = e x ( ln ( x + y ) e y ln ( x ) ) y = ln ( x + y ) e x e y ln ( x ) e x y =\frac{e^x(\ln(x+y)e^y-\ln(x))}{y}=\frac{\ln(x+y)e^xe^y-\ln(x)e^x}{y} = ln ( x + y ) e x + y ln ( x ) e x y =\frac{\ln(x+y)e^{x+y}-\ln(x)e^x}{y} take the limit: lim y 0 ln ( x + y ) e x + y ln ( x ) e x y \lim_{y\to0}\frac{\ln(x+y)e^{x+y}-\ln(x)e^x}{y} note that using the definition of the derivative, this limit is the same as d d x ( ln ( x ) e x ) = ln ( x ) e x + e x x \frac{d}{dx}(\ln(x)e^x)=\ln(x)e^x+\frac{e^x}{x} taking the other limit gives: lim x 1 ( ln ( x ) e x + e x x ) \lim_{x\to1}(\ln(x)e^x+\frac{e^x}{x}) = ln ( 1 ) e 1 + e 1 1 =\ln(1)e^1+\frac{e^1}{1} = 0 + e = e . =0+e=\boxed{e}.

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