The ebook Elementary Calculus is based on material originally written by H.J. Keisler. For more information please read the copyright pages.


L'Hospitals Rule for ∞/∞

L'HOSPITAL'S RULE FOR ∞/∞

Suppose c is a real number, and in some deleted neighborhood of c, f'(x) and g'(x) exist and g'(x) ≠ 0. Assume that

limx→c f(x) = ∞, limx→c g(x) = ∞.

If 05_limits_g_approx-115.gif exists or is infinite, then

05_limits_g_approx-116.gif

The rule for ∞/∞ is exactly the same, word for word, as the rule for 0/0, except that 0 is replaced by ∞. We omit the proof, which is more difficult in the case ∞/∞. Actually, the assumption

limx→c f(x) = ∞

is not needed.

Again, l'Hospital's Rule for ∞/∞ also holds for the other types of limits,

x → c+, x → c-, x → ∞, x → -∞.

Example 6
Example 7


Last Update: 2006-11-14