تمارين - 2BACSEF

التانية باكالوريا العلوم التجريبية – خيار فرنسي


درس : Fonction logarithme népérien

Exercice 10

Calculer les limites suivantes

1/ limx0+(1xln(x))\quad\lim\limits_{x\to0^+}\left(\dfrac{1}{x}-\ln(x)\right)

2/ limx0ln(x+1)ln(1x)x\quad\lim\limits_{x\to0}\dfrac{\ln(x+1)-\ln(1-x)}{x}

3/ limx+ln(x2+2)x+2\quad\lim\limits_{x\to+\infty}\dfrac{\ln(x^2+2)}{x+2}

1/ limx0+(1xlnx)=+\quad\lim\limits_{x \to 0^+} \left(\dfrac{1}x- \ln x \right)=+\infty

car {limx0+1x=+ limx0+lnx=\Bigg\{ \begin{array}{l} \lim\limits_{x \to 0^+} \dfrac{1}{x} = +\infty \\~\\ \lim\limits_{x \to 0^+} \ln x = -\infty \end{array}

2/ limx0ln(1+x)ln(1x)x="00"    F.I\quad \lim\limits_{x \to 0} \frac{\ln(1+x) - \ln(1-x)}{x}=\quad "\dfrac{0}{0}" ~~~~\text{F.I}

limx0ln(1+x)ln(1x)x=limx0ln(1+x)x+ln(1x)x=1+1=2\begin{aligned} \bullet\quad&\lim\limits_{x \to 0} \frac{\ln(1+x) - \ln(1-x)}{x} \\ &= \lim\limits_{x \to 0} \frac{\ln(1+x)}{x} + \frac{\ln(1-x)}{-x} \\ &= 1 + 1 = 2 \end{aligned}

car {limx0ln(1+x)x=1 limx0ln(1x)x=limt0ln(1+t)t=1\Bigg\{ \begin{array}{l} \lim\limits_{x \to 0} \frac{\ln(1+x)}{x} = 1 \\~\\ \lim\limits_{x \to 0} \frac{\ln(1-x)}{-x} = \lim\limits_{t \to 0} \frac{\ln(1+t)}{t} = 1 \end{array}

3/ limx+ln(x2+2)x+2\quad\lim\limits_{x\to+\infty}\dfrac{\ln(x^2+2)}{x+2}

=limx+ln[x2(1+2x2)]x+2 =limx+ln(x2)+ln(1+2x2)x+2 =limx+2ln(x)x+2+ln(1+2x2)x+2 =limx+2ln(x)x.xx+2+ln(1+2x2)x+2 =0×2+0=0\begin{aligned} &=\lim\limits_{x\to+\infty} \dfrac{\ln\left[x^2\left(1+\dfrac2{x^2}\right)\right]}{x+2} \\~\\ &=\lim\limits_{x\to+\infty} \dfrac{\ln(x^2)+\ln\left(1+\dfrac{2}{x^2}\right)}{x+2} \\~\\ &=\lim\limits_{x\to+\infty} \dfrac{2\ln(x)}{x+2}+\dfrac{\ln\left(1+\dfrac{2}{x^2}\right)}{x+2} \\~\\ &=\lim\limits_{x\to+\infty} \dfrac{2\ln(x)}{x}.\dfrac{x}{x+2}+\dfrac{\ln\left(1+\dfrac{2}{x^2}\right)}{x+2} \\~\\ &=0\times2+0=0 \end{aligned}

car :

{limx+lnxx=0 limx+2xx+2=limx+2xx=2 limx+2x2=0\left\{ \begin{align*} &\lim\limits_{x\to+\infty} \dfrac{\ln x}{x}=0 \\~\\ &\lim\limits_{x\to+\infty} \dfrac{2x}{x+2} =\lim\limits_{x\to+\infty}\dfrac{2x}x=2 \\~\\ &\lim\limits_{x\to+\infty} \dfrac{2}{x^2}=0 \end{align*} \right.