What is the Limit?

Calculus Level 2

lim x ( log ( 2 + 1 x 2 ) + log ( 1 4 + 1 x 2 ) ) ? \lim_{x\rightarrow \infty}\hspace{1mm}\left(\log(2+\frac{1}{x^2})+\log(\frac{1}{4+\frac{1}{x^2}})\right)\approx\, ?

−0.301 The limit does not exist −0.125 1

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1 solution

Drex Beckman
Jan 21, 2016

A property of logarithms: l o g ( x ) + l o g ( y ) = l o g ( x y ) log(x)+log(y)=log(x\cdot y) . That being said: l o g ( 2 + 1 x 2 ) + l o g ( 1 4 + 1 x 2 ) = l o g ( 2 + 1 x 2 1 4 + 1 x 2 ) log(2+\frac{1}{x^2})+log(\frac{1}{4+\frac{1}{x^2}})=log(2+\frac{1}{x^2}\cdot{\frac{1}{4+\frac{1}{x^2}}}) This becomes: l o g ( 2 + 1 x 2 4 + 1 x 2 ) log(\frac{2+\frac{1}{x^2}}{4+\frac{1}{x^2}}) . We can find the number by taking the limit as x approaches infinity: lim x l o g ( 2 + 1 x 2 4 + 1 x 2 ) \lim_{x\rightarrow \infty}\hspace{1mm}log(\frac{2+\frac{1}{x^2}}{4+\frac{1}{x^2}}) As x x\rightarrow \infty , 1 x 2 0 \frac{1}{x^2} \rightarrow 0 . Therefore, we are left with: l o g ( 2 4 ) = l o g ( 0.5 ) 0.301 log(\frac{2}{4})=log(0.5)\approx \hspace{0.5mm} -0.301

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