x → 0 + lim ( 1 + sin ( 4 x ) ) cot ( x )
Evaluate the limit above.
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f ( x ) , g ( x ) → 1 , ∞ lim ( f ( x ) ) g ( x ) = e lim ( f ( x ) − 1 ) ( g ( x ) )
Using the same formula, we get the limit as e tan x sin 4 x = e 4
Let y = ( 1 + s i n ( 4 x ) ) c o t ( x )
Since direct substitution results in an indeterminate form 1 i n f
we can take natural log of this equation
l n ( y ) = c o t ( x ) ∗ l n ( 1 + s i n ( 4 x ) )
l n ( y ) = l n ( 1 + s i n ( 4 x ) ) / ( 1 / c o t ( x ) )
l n ( y ) = l n ( 1 + 4 s i n ( x ) ) / t a n ( x )
Since the answer is still indeterminate as 0/0
Apply L'Hospitals rule
4 c o s ( x ) / ( 1 + 4 s i n x ) ∗ s e c 2 x
4 c o s 3 ( x ) / ( 1 + 4 s i n x )
Substituting x=0
4/1
ln(y) when x approaches 0=4
*y=e^4 ,the limit of function. *
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Relevant wiki: L'Hopital's Rule - Basic
L = x → 0 + lim ( 1 + sin ( 4 x ) ) cot x = exp ( x → 0 + lim cot x sin ( 4 x ) ) = exp ( x → 0 + lim tan x sin ( 4 x ) ) = exp ( x → 0 + lim sec 2 x 4 cos ( 4 x ) ) = e 4 A 1 ∞ case, the following applies x → a lim ( f ( x ) ) h ( x ) = exp ( x → a lim h ( x ) ( f ( x ) − 1 ) ) A 0/0 case, L’H o ˆ pital’s rule applies Differentiate up and down w.r.t. x
Reference : 1 ∞ limit (see: 2nd method )