# ilimts w/ result being infinity minus infinity

• March 17th 2010, 05:09 AM
Amberosia32
ilimts w/ result being infinity minus infinity
I solved had two limits and I was adding them together, the fist come out to infinity and the second negative infinity giving me infinity - infinity What is that? I know normally you use l'hopital's rule but since it's two limits I don't know what to do.
Limit A= infinity and Limit B=negative infinity The question is what is limit A+limit B I would post the exact problem but It might be graded.
• March 17th 2010, 06:12 AM
General
Quote:

Originally Posted by Amberosia32
I solved had two limits and I was adding them together, the fist come out to infinity and the second negative infinity giving me infinity - infinity What is that? I know normally you use l'hopital's rule but since it's two limits I don't know what to do.

Post the problem !!
How in earth we will know how to solve these limits if we did not see them ?
• March 17th 2010, 06:18 AM
HallsofIvy
In general, if f(x)- g(x) goes to "infinity- infinity", then $e^{f(x)- g(x)}= \frac{e^{f(x)}}{e^{g(x)}}$ goes to "infinity over infinity" and you can apply L'Hopital's rule directly. If the imit of that is L then the limit of the original is ln(L).

However General's point is well taken- a specific limit problem may have special properties that make it easier than the general situation. Typically, multiplying "numerator and denominator" by f(x)+ g(x) will help.