Guillaume De l'Hôpital day 1 L’Hôpital’s Rule Actually, L’Hôpital’s Rule was developed by his teacher Johann Bernoulli. De l’Hôpital paid Bernoulli for private lessons, and then published the first Calculus book based on those lessons. Greg Kelly, Hanford High School, Richland, Washington

Johann Bernoulli day 1 L’Hôpital’s Rule

Zero divided by zero can not be evaluated, and is an example of indeterminate form. Consider: If we try to evaluate this by direct substitution, we get: In this case, we can evaluate this limit by factoring and canceling:

If we zoom in far enough, the curves will appear as straight lines. The limit is the ratio of the numerator over the denominator as x approaches 2.

As becomes:

As becomes:

L’Hôpital’s Rule: If is indeterminate, then:

We can confirm L’Hôpital’s rule by working backwards, and using the definition of derivative:

Example: If it’s no longer indeterminate, then STOP! If we try to continue with L’Hôpital’s rule: which is wrong, wrong, wrong!

On the other hand, you can apply L’Hôpital’s rule as many times as necessary as long as the fraction is still indeterminate: not (Rewritten in exponential form.)

L’Hôpital’s rule can be used to evaluate other indeterminate forms besides. The following are also considered indeterminate: The first one,, can be evaluated just like. The others must be changed to fractions first.

This approaches We already know that but if we want to use L’Hôpital’s rule:

If we find a common denominator and subtract, we get: Now it is in the form This is indeterminate form L’Hôpital’s rule applied once. Fractions cleared. Still

L’Hôpital again.

Indeterminate Forms: Evaluating these forms requires a mathematical trick to change the expression into a fraction. When we take the log of an exponential function, the exponent can be moved out front. We can then write the expression as a fraction, which allows us to use L’Hôpital’s rule. We can take the log of the function as long as we exponentiate at the same time. Then move the limit notation outside of the log.

Indeterminate Forms: L’Hôpital applied Example:

A special symbol for L’Hôpital’s rule : Note the letter “H” over the equal sign.  Some textbooks (and some college professors) use a special form of the equal sign when L’Hôpital’s rule is applied: Our textbook does not use this symbol, but be aware that you may see it in the future. You may also see the letters L’H written over the equal sign, which means the same thing.