Sunday, October 28, 2012

What is the limit as x=>0 of sin(x)/x+tan(x) ?

It is not clear if the denominator of the ratio is just x
or is (x+tan x).


This thing must be specified with the help
of the brackets.


If the denominator of the ratio is
x:


lim [sin(x)/x+tan(x)] = lim [sin(x)/x] + lim
tan(x)


lim [sin(x)/x] is an elementary limit and the result
is1.


We'll calculate lim tan
x.


We'll substitute x by the value
0.


lim tan x = tan 0 = 0


So,
the limit is:


lim [sin(x)/x+tan(x)] = 1 + 0 =
1



Now, if the denominator is
(x+tan x), we'll calculate the limit:


lim
[sin(x)/(x+tanx)]  = sin 0/(x + tan 0)


lim
[sin(x)/(x+tanx)]  = 0/0


"0/0" is an
indetermination


We'll use l'hospital rule. We'll
differentiate separately numerator and denominator.


(sinx)'
= cos x


(x+tanx)' = 1 + 1/(cos
x)^2


lim [sin(x)/(x+tanx)]  = lim
(sinx)'/(x+tanx)'


lim (sinx)'/(x+tanx)' = lim cos x/[1 +
1/(cos x)^2]


We'll substitute x by
0:


lim cos x/[1 + 1/(cos x)^2] =cos 0/[1 + 1/(cos
0)^2]


cos 0/[1 + 1/(cos 0)^2] = 1/(1+1) =
1/2


lim
[sin(x)/(x+tanx)]  =
1/2

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