l'Hôpital's Rule (DP IB Analysis & Approaches (AA)): Revision Note

l'Hôpital's rule

What is l’Hôpital’s Rule?

• l’Hôpital’s rule is a method involving calculus that allows you to find the value of certain limits

• Specifically, it allows you to attempt to evaluate the limit of a quotient  for which the usual limit evaluation techniques would return one of the indeterminate forms  or .

How do I evaluate a limit using l’Hôpital’s Rule?

• STEP 1
  Check that the limit of the quotient results in one of the indeterminate forms given above

  • I.e., check that or  

• STEP 2
  Find the derivatives of the numerator and denominator of the quotient
   

• STEP 3
  Check whether the limit  exists
   

• STEP 4
  If that limit does exist, then 

• STEP 5
  If or  then you may repeat the process by considering  (and possibly higher order derivatives after that)

  • As long as the limits continue giving indeterminate forms you may continue applying l’Hôpital’s rule

  • Each time this happens find the next set of derivatives and consider the limit again