# L’Hopital’s Rule is a Hospital to Cure Ailing Functions

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## Extending L’ Hôpital’s Rule

L’ Hôpital’s Rule is transitive: if the first derivatives f'(x)/g'(x) do not help, perhaps the second derivatives f”(x)/g”(x) will resolve the issue. Or the third, or the fourth…

## Limits to L’ Hôpital’s Rule

One nasty problem comes if the values for f'(x)/g'(x) oscillate as ‘x’ approaches the critical value; for example if the sign changes between positive and negative.

In other cases, the limit of f(x)/g(x) may seem bounded even though each approaches infinity, but f'(x)/g'(x) might head for infinity.

The main limitation is that the limit of f(x)/g(x) must be “zero divided by zero” or “infinity divided by infinity”.

## The Men Behind L’ Hôpital’s Rule

Although only one person’s title graces “L’ Hôpital’s Rule,” in fact two mathematicians contributed to it.

## Guillaume François Antoine, Marquis de l’ Hôpital

Picture of Guillaume, Marquis de l’Hôpital by Bemoeial2

L’Hôpital was born in Paris in 1661. As a son of a noble in a high-ranking military family, he entered the army. He resigned as captain of a cavalry regiment due to severe myopia, but had previously shown his talent for mathematics.

At fifteen years of age, he had already solved several problems developed by Blaise Pascal. Although he had continued to study mathematics while enrolled, he was free to fully pursue mathematics after leaving the army. He met, befriended and studied under Jean Bernoulli, one of the very rare breed of folk who understood the newfangled “calculus” of Newton and Leibniz. Many historians believe that l’Hôpital’s references to Bernoulli’s ideas regarding L’Hôpital’s Rule indicate that Bernoulli actually had the original idea.

He was made a member of the Academy of Sciences of Paris and wrote a number of works explaining and developing calculus. L’Hôpital died in 1704.

## Jean Bernoulli, aka John Bernoulli

Picture of Jean Bernoulli by Magnus Manske

Jean Bernoulli, born in 1667, earned a philosophy degree and then a medical license at the University of Basel in Switzerland. A member of a family famed for its mathematicians and scientists, he continued the family tradition by teaching the Marquis de l’Hôpital in Paris. On l’Hôpital’s recommendation, he was hired at Gröningen in the Netherlands. Later he took his deceased brother’s place as a professor of mathematics in Basel.

Bernoulli was seen as instrumental in helping develop L’ Hôpital’s Rule. However, he is perhaps most famous for one incident involving several great mathematicians. He posed a difficult challenge, the “brachistochrone” (“quickest time”) about how quickly a bead would slide down a string not dangled vertically and what shape the string would take. Correct answers came from the Bernoulli brothers, l’ Hôpital, Leibniz and an anonymous respondent. Jean Bernoulli identified that correspondent as Isaac Newton, because “I recognize the lion by his paw.”

The solution to the problem, by the way, is “an upside-down cycloid.” But that might be the topic of another article.

## The Value of L’ Hôpital’s Rule

L’ Hôpital’s Rule is a very useful tool, when properly used, to find the limit of a function at a point where the function degenerates to either “zero over zero” or “infinity over infinity”. There are limits to its capabilities, even though its capabilities are derived from limits.

References:
Fox, W. Guillaume-François-Antoine de L’Hôpital. The Catholic Encyclopedia. Vol. 7. New York: Robert Appleton Company. (1910). Accessed Aug. 13, 2011.

Weisstein, E. L’Hospital’s Rule. MathWorld.  Accessed Aug. 13, 2011.

Integration by Parts. Harvey Mudd College. Accessed Aug. 13, 2011.

John Bernoulli. McGraw-Hill Higher Education. Accessed Aug. 13, 2011.

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