L'HOSPITAL'S RULE (WITH EXAMPLES)
L'HOSPITAL'S
RULE
IF Lim F(X)/G(X) = 0/0 ;
X->a
THEN Lim F(X)/G(X) = Lim F'(X)/G'(X)
X->a X->a
PROBLEMS BASED ON ABOVE RULE:
_
1. Lim sin(1-x)/√x - 1 = ?
x->1
ANSWER:
IF WE PUT x=1; WE GET ........... sin(1-x)
= sin(1-1)
=sin(0)
_ =0
AND ..................√x - 1
=1-1
=0
SO, _
sin(1-x)/√x - 1 = 0/o
SO APPLY L'HOSPITAL'S RULE;
_ _
Lim sin(1-x)/√x - 1 = Lim d/dx( sin(1-x)) / d/dx( √x - 1 )
x->1 x->1
_
= Lim cos(1-x). (-1) / 1/2√x
x->1
IF WE PUT x=1; cos(1-x)_=cos(1-1)=cos0=1.
and √x = √1 =1.
= -1 / (1/2)
= -2
_
so; Lim sin(1-x)/√x - 1 = -2
x->1
2. Lim sin(5x) / ln(1+4x)=?
x->0
ANS: If we put x=0
sin(5x)=sin(5 . 0)=sin0=0
ln(1+4x)=ln(1+ 4 . 0)= ln(1 + 0)=ln(1)=0
SO; Lim sin(5x) / ln(1+4x)=0/0
x->0
HENCE WE SHOULD APPLY L'HOSPITAL'S RULE:
Lim sin(5x) / ln(1+4x)=Lim d/dx( sin5x) / d/dx( ln(1+4x) )
X->0 x->0
=Lim (cos5x). 5 / (4/1+4x)
x->0
IF WE PUT x=0
= cos(5.0).5 / (4/1+0)
= cos0 . 5 /4
= 5/4
HENCE; Lim sin(5x) / ln(1+4x) = 5/4.
X->0
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