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Joint Entrance Examination
Mathematics
Limits, Continuity and Differentiability
Question 172
Question 172 of 401
Medium
NUMERICAL
If the function
f
f
f
defined on
(
−
1
3
,
1
3
)
\left(-\dfrac13,\dfrac13\right)
(
−
3
1
,
3
1
)
by
f
(
x
)
=
{
1
x
log
e
(
1
+
3
x
1
−
2
x
)
,
x
≠
0
k
,
x
=
0
f(x)=\begin{cases}\dfrac1x\log_e\left(\dfrac{1+3x}{1-2x}\right), & x\ne0\\k, & x=0\end{cases}
f
(
x
)
=
⎩
⎨
⎧
x
1
lo
g
e
(
1
−
2
x
1
+
3
x
)
,
k
,
x
=
0
x
=
0
is continuous, then
k
k
k
is equal to \_\_\_\_\_\_\_. Answer: 5
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Official solution
\[\lim_{x\to0}\frac{\ln(1+3x)}{3x}\cdot3-\frac{\ln(1-2x)}{-2x}\cdot(-2)=3+2=5\Rightarrow k=5\]
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