LIMITS OF RATIONAL FUNCTIONS

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Rational function is the ratio of two polynomial functions and it is usually represented as

where g(x) ≠ 0.

Consider the following limit of a rational function.

When you evaluate the above the limit of a rational function, you may get one of the following results.

Case 1 :

The above limit exists and it is equal to the finite value received in the last step of evaluation.

Case 2 :

Dividing a non-zero number by zero is undefined. So, the above limit does not exists.

Case 3 :

Dividing zero by zero is called indeterminate form (not undefined). Indeterminate form is not a final answer for the limit of a rational function. We can evaluate such limit by simiplifying the given rational function.

The following steps will be useful to evaluate the limit of a rational function which initially results the inderterninate form 'zero by zero'.

Step 1 :

Factor the polynomials in numerartor and denominator.

Step 2 :

Cancel out the common factors found in numerator and denominator (simplification).

Step 3 :

Substitute the given limit for the variable x and evaluate. The answer will be a finite value.

Solved Problems

Evaluate the following limits :

Problem 1 :

Solution :

Problem 2 :

Solution :

Since the evaluation of the given limit is undefined, the limit does not exist. 

Problem 3 :

Solution :

Problem 4 :

Solution :

Since the evaluation of the given limit results indeterminate form 'zero by zero', simplify the rational function and substitute the given limit for x and evaluate.

Note :

When you evaluate limits of rational functions, it is advisable to simplify the rational function first (if possible), then substitute the given limit for the variable.

Problem 5 :

Solution :

Use the following algebraic identity and factor the expression in denominator.

a2 - b2 = (a + b)(a - b)

Problem 6 :

Solution :

Use the following algebraic identity and factor the expression in denominator.

a3 + b3 = (a + b)(a2 - ab + b2)

Problem 7 :

Solution :

Problem 8 :

Solution :

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