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Retour au chapitre
Fonction logarithme népérien
Les limites avec la fonction
x
↦
ln
(
x
)
x\mapsto \ln \left(x\right)
x
↦
ln
(
x
)
- Exercice 2
15 min
30
Déterminer les limites suivantes :
Question 1
lim
x
→
0
+
ln
(
x
)
+
3
\mathop{\lim }\limits_{x\to 0^{+} } \ln \left(x\right)+3
x
→
0
+
lim
ln
(
x
)
+
3
que l'on peut aussi écrire
lim
x
→
0
x
>
0
ln
(
x
)
+
3
\mathop{\lim }\limits_{\begin{array}{l} {x\to 0} \\ {x>0} \end{array}} \ln \left(x\right)+3
x
→
0
x
>
0
lim
ln
(
x
)
+
3
Correction
lim
x
→
0
+
ln
(
x
)
=
−
∞
\lim\limits_{x\to 0^{+} } \ln \left(x\right)=-\infty
x
→
0
+
lim
ln
(
x
)
=
−
∞
lim
x
→
0
+
ln
(
x
)
=
−
∞
lim
x
→
0
+
3
=
3
}
\left. \begin{array}{ccc} {\lim\limits_{x\to 0^{+} }\ln \left(x\right)} & {=} & {-\infty } \\ {\lim\limits_{x\to 0^{+} } 3} & {=} & {3 } \end{array}\right\}
x
→
0
+
lim
ln
(
x
)
x
→
0
+
lim
3
=
=
−
∞
3
}
par addition
\text{\red{par addition}}
par addition
lim
x
→
0
+
ln
(
x
)
+
3
=
−
∞
\mathop{\lim }\limits_{x\to 0^{+} } \ln \left(x\right)+3=-\infty
x
→
0
+
lim
ln
(
x
)
+
3
=
−
∞
Question 2
lim
x
→
0
+
3
ln
(
x
)
−
2
\mathop{\lim }\limits_{x\to 0^{+} } 3\ln \left(x\right)-2
x
→
0
+
lim
3
ln
(
x
)
−
2
Correction
lim
x
→
0
+
ln
(
x
)
=
−
∞
\lim\limits_{x\to 0^{+} } \ln \left(x\right)=-\infty
x
→
0
+
lim
ln
(
x
)
=
−
∞
lim
x
→
0
+
3
ln
(
x
)
=
−
∞
lim
x
→
0
+
−
2
=
−
2
}
\left. \begin{array}{ccc} {\lim\limits_{x\to 0^{+} }3\ln \left(x\right)} & {=} & {-\infty } \\ {\lim\limits_{x\to 0^{+} } -2} & {=} & {-2 } \end{array}\right\}
x
→
0
+
lim
3
ln
(
x
)
x
→
0
+
lim
−
2
=
=
−
∞
−
2
}
par addition
\text{\red{par addition}}
par addition
lim
x
→
0
+
3
ln
(
x
)
−
2
=
−
∞
\mathop{\lim }\limits_{x\to 0^{+} } 3\ln \left(x\right)-2=-\infty
x
→
0
+
lim
3
ln
(
x
)
−
2
=
−
∞
Question 3
lim
x
→
0
+
5
ln
(
x
)
+
3
x
\mathop{\lim }\limits_{x\to 0^{+} } 5\ln \left(x\right)+3x
x
→
0
+
lim
5
ln
(
x
)
+
3
x
Correction
lim
x
→
0
+
ln
(
x
)
=
−
∞
\lim\limits_{x\to 0^{+} } \ln \left(x\right)=-\infty
x
→
0
+
lim
ln
(
x
)
=
−
∞
lim
x
→
0
+
5
ln
(
x
)
=
−
∞
lim
x
→
0
+
3
x
=
0
}
\left. \begin{array}{ccc} {\lim\limits_{x\to 0^{+} }5\ln \left(x\right)} & {=} & {-\infty } \\ {\lim\limits_{x\to 0^{+} } 3x} & {=} & {0 } \end{array}\right\}
x
→
0
+
lim
5
ln
(
x
)
x
→
0
+
lim
3
x
=
=
−
∞
0
}
par addition
\text{\red{par addition}}
par addition
lim
x
→
0
+
5
ln
(
x
)
+
3
x
=
−
∞
\mathop{\lim }\limits_{x\to 0^{+} } 5\ln \left(x\right)+3x=-\infty
x
→
0
+
lim
5
ln
(
x
)
+
3
x
=
−
∞
Question 4
lim
x
→
0
+
−
2
ln
(
x
)
−
7
x
+
1
\mathop{\lim }\limits_{x\to 0^{+} } -2\ln \left(x\right)-7x+1
x
→
0
+
lim
−
2
ln
(
x
)
−
7
x
+
1
Correction
lim
x
→
0
+
ln
(
x
)
=
−
∞
\lim\limits_{x\to 0^{+} } \ln \left(x\right)=-\infty
x
→
0
+
lim
ln
(
x
)
=
−
∞
2
°
)
C
a
l
c
u
l
o
n
s
d
a
n
s
u
n
s
e
c
o
n
d
t
e
m
p
s
l
a
l
i
m
i
t
e
d
e
−
2
ln
(
x
)
‾
\underline{\color{black}2°)\;Calculons\;dans\;un\;second\;temps\;la\;limite\;de\;-2\ln(x)}
2°
)
C
a
l
c
u
l
o
n
s
d
an
s
u
n
seco
n
d
t
e
m
p
s
l
a
l
imi
t
e
d
e
−
2
l
n
(
x
)
lim
x
→
0
+
ln
(
x
)
=
−
∞
lim
x
→
0
+
−
2
=
−
2
}
\left. \begin{array}{ccc} {\lim\limits_{x\to 0^{+} }\ln \left(x\right)} & {=} & {-\infty } \\ {\lim\limits_{x\to 0^{+} } -2} & {=} & {-2 } \end{array}\right\}
x
→
0
+
lim
ln
(
x
)
x
→
0
+
lim
−
2
=
=
−
∞
−
2
}
par produit
\text{\red{par produit}}
par produit
lim
x
→
0
+
−
2
ln
(
x
)
=
+
∞
\mathop{\lim }\limits_{x\to 0^{+} } -2\ln \left(x\right)=+\infty
x
→
0
+
lim
−
2
ln
(
x
)
=
+
∞
O
n
p
e
u
t
d
o
n
c
c
o
n
c
l
u
r
e
:
‾
\underline{\color{black}On\;peut\;donc\;conclure\;:}
O
n
p
e
u
t
d
o
n
c
co
n
c
l
u
re
:
lim
x
→
0
+
−
2
ln
(
x
)
=
+
∞
lim
x
→
0
+
−
7
x
+
1
=
1
}
\left. \begin{array}{ccc} {\lim\limits_{x\to 0^{+} }-2\ln \left(x\right)} & {=} & {+\infty } \\ {\lim\limits_{x\to 0^{+} } -7x+1} & {=} & {1 } \end{array}\right\}
x
→
0
+
lim
−
2
ln
(
x
)
x
→
0
+
lim
−
7
x
+
1
=
=
+
∞
1
}
par somme
\text{\red{par somme}}
par somme
lim
x
→
0
+
−
2
ln
(
x
)
−
7
x
+
1
=
+
∞
\mathop{\lim }\limits_{x\to 0^{+} } -2\ln \left(x\right)-7x+1=+\infty
x
→
0
+
lim
−
2
ln
(
x
)
−
7
x
+
1
=
+
∞
Question 5
lim
x
→
0
+
x
ln
(
x
)
−
4
\mathop{\lim }\limits_{x\to 0^{+} } x\ln \left(x\right)-4
x
→
0
+
lim
x
ln
(
x
)
−
4
Correction
lim
x
→
0
+
x
ln
(
x
)
=
0
\lim\limits_{x\to 0^{+} } x\ln \left(x\right)=0
x
→
0
+
lim
x
ln
(
x
)
=
0
lim
x
→
0
+
x
ln
(
x
)
=
0
lim
x
→
0
+
−
4
=
−
4
}
\left. \begin{array}{ccc} {\lim\limits_{x\to 0^{+} }x\ln \left(x\right)} & {=} & {0 } \\ {\lim\limits_{x\to 0^{+} } -4} & {=} & {-4 } \end{array}\right\}
x
→
0
+
lim
x
ln
(
x
)
x
→
0
+
lim
−
4
=
=
0
−
4
}
par addition
\text{\red{par addition}}
par addition
lim
x
→
0
+
x
ln
(
x
)
−
4
=
−
4
\mathop{\lim }\limits_{x\to 0^{+} } x\ln \left(x\right)-4=-4
x
→
0
+
lim
x
ln
(
x
)
−
4
=
−
4
Question 6
lim
x
→
0
+
x
ln
(
x
)
+
6
x
+
8
\mathop{\lim }\limits_{x\to 0^{+} } x\ln \left(x\right)+6x+8
x
→
0
+
lim
x
ln
(
x
)
+
6
x
+
8
Correction
lim
x
→
0
+
x
ln
(
x
)
=
0
\lim\limits_{x\to 0^{+} } x\ln \left(x\right)=0
x
→
0
+
lim
x
ln
(
x
)
=
0
lim
x
→
0
+
x
ln
(
x
)
=
0
lim
x
→
0
+
6
x
+
8
=
8
}
\left. \begin{array}{ccc} {\lim\limits_{x\to 0^{+} }x\ln \left(x\right)} & {=} & {0 } \\ {\lim\limits_{x\to 0^{+} } 6x+8} & {=} & {8 } \end{array}\right\}
x
→
0
+
lim
x
ln
(
x
)
x
→
0
+
lim
6
x
+
8
=
=
0
8
}
par addition
\text{\red{par addition}}
par addition
lim
x
→
0
+
x
ln
(
x
)
+
6
x
+
8
=
8
\mathop{\lim }\limits_{x\to 0^{+} } x\ln \left(x\right)+6x+8=8
x
→
0
+
lim
x
ln
(
x
)
+
6
x
+
8
=
8
Question 7
lim
x
→
0
+
ln
(
x
+
1
)
x
+
2
\mathop{\lim }\limits_{x\to 0^{+} } \frac{\ln \left(x+1\right)}{x} +2
x
→
0
+
lim
x
ln
(
x
+
1
)
+
2
Correction
lim
x
→
0
+
ln
(
x
+
1
)
x
=
1
\lim\limits_{x\to 0^{+} } \frac{\ln \left(x+1\right)}{x}=1
x
→
0
+
lim
x
ln
(
x
+
1
)
=
1
lim
x
→
0
+
ln
(
x
+
1
)
x
=
1
lim
x
→
0
+
2
=
2
}
\left. \begin{array}{ccc} {\lim\limits_{x\to 0^{+} }\frac{\ln \left(x+1\right)}{x}} & {=} & {1 } \\ {\lim\limits_{x\to 0^{+} } 2} & {=} & {2 } \end{array}\right\}
x
→
0
+
lim
x
l
n
(
x
+
1
)
x
→
0
+
lim
2
=
=
1
2
⎭
⎬
⎫
par addition
\text{\red{par addition}}
par addition
lim
x
→
0
+
ln
(
x
+
1
)
x
+
2
=
3
\mathop{\lim }\limits_{x\to 0^{+} } \frac{\ln \left(x+1\right)}{x} +2=3
x
→
0
+
lim
x
ln
(
x
+
1
)
+
2
=
3
Question 8
lim
x
→
0
+
x
ln
(
x
)
−
7
x
\mathop{\lim }\limits_{x\to 0^{+} } x\ln \left(x\right)-7x
x
→
0
+
lim
x
ln
(
x
)
−
7
x
.
Correction
lim
x
→
0
+
x
ln
(
x
)
=
0
\lim\limits_{x\to 0^{+} } x\ln \left(x\right)=0
x
→
0
+
lim
x
ln
(
x
)
=
0
lim
x
→
0
+
x
ln
(
x
)
=
0
lim
x
→
0
+
−
7
x
=
0
}
\left. \begin{array}{ccc} {\lim\limits_{x\to 0^{+} }x\ln \left(x\right)} & {=} & {0 } \\ {\lim\limits_{x\to 0^{+} } -7x} & {=} & {0} \end{array}\right\}
x
→
0
+
lim
x
ln
(
x
)
x
→
0
+
lim
−
7
x
=
=
0
0
}
par addition
\text{\red{par addition}}
par addition
lim
x
→
0
+
x
ln
(
x
)
−
7
x
=
0
\mathop{\lim }\limits_{x\to 0^{+} } x\ln \left(x\right)-7x=0
x
→
0
+
lim
x
ln
(
x
)
−
7
x
=
0