GRE Subject Test: Math : Defining Derivatives with Limits

Study concepts, example questions & explanations for GRE Subject Test: Math

varsity tutors app store varsity tutors android store

Example Questions

Example Question #13 : Finding Derivatives

Evaluate: \(\displaystyle \lim _{x \to 2} \frac {x^2-4}{x^3-8}\)

Possible Answers:

\(\displaystyle \frac {1}{3}\)

\(\displaystyle -3\)

Does Not Exist

\(\displaystyle -\frac {1}{3}\)

Correct answer:

\(\displaystyle \frac {1}{3}\)

Explanation:

Step 1: Try plugging in \(\displaystyle x=2\) into the denominator of the function. We want to make sure that the bottom does not become \(\displaystyle 0\)...

\(\displaystyle 2^3-8=8-8=0\).. We got zero, and we cannot have zero in the denominator. So, we must try and factor the function (numerator and denominator):

Step 2: Factor:

\(\displaystyle \frac {(x+2)(x-2)}{(x-2)(x^2+2x+4)}\)

Step 3: Reduce:

\(\displaystyle \frac {x+2}{x^2+2x+4}\)

Step 4: Now that we got rid of the factor that made the denominator zero, we know that this function has a limit.

Step 5: Plug in \(\displaystyle x=2\) into the reduced factor form:

Simplify as much as possible...


\(\displaystyle \frac {2+2}{2^2+2(2)+4}=\frac {4}{12}=\frac {1}{3}\)

The limit of this function as x approaches \(\displaystyle 2\) is \(\displaystyle \frac {1}{3}\)

Learning Tools by Varsity Tutors