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Have you ever seen fractions in mathematics? Well, rational functions are similar to fractions, but more interesting because they involve variables!
A rational function is a function in the form of a fraction, where both the numerator and denominator are polynomial functions. Simply put, a rational function can be written as:
P ( x ) P(x) P ( x ) is a polynomial in the numerator
Q ( x ) Q(x) Q ( x ) is a polynomial in the denominator
Q ( x ) ≠ 0 Q(x) \neq 0 Q ( x ) = 0 (denominator cannot be zero)
Let's look at a real example to better understand rational functions.
Mr. Budi wants to build a rectangular chicken coop with an area of 100 m 2 100 \text{ m}^2 100 m 2 . He wants to know the relationship between the length and width of the coop.
If the length of the coop is x meters x \text{ meters} x meters , then:
Area = length × width = 100 \text{Area} = \text{length} \times \text{width} = 100 Area = length × width = 100
x × width = 100 x \times \text{width} = 100 x × width = 100
Width is 100 x \frac{100}{x} x 100
The function f ( x ) = 100 x f(x) = \frac{100}{x} f ( x ) = x 100 is an example of a rational function!
The simplest form of a rational function:
Where k k k is a constant. Example: f ( x ) = 5 x f(x) = \frac{5}{x} f ( x ) = x 5
Both numerator and denominator are linear functions:
Example: f ( x ) = 2 x + 3 x − 1 f(x) = \frac{2x + 3}{x - 1} f ( x ) = x − 1 2 x + 3
Involves quadratic functions in the numerator or denominator:
The domain of a rational function is all values of x x x that make the function defined. Remember, the denominator cannot be zero!
Find values of x x x that make the denominator = 0 = 0 = 0
The domain is all real numbers except those values
Example: Determine the domain of f ( x ) = x + 2 x − 3 f(x) = \frac{x + 2}{x - 3} f ( x ) = x − 3 x + 2
Denominator is zero when: x − 3 = 0 x - 3 = 0 x − 3 = 0
So: x = 3 x = 3 x = 3
Domain: D f = { x ∣ x ≠ 3 , x ∈ R } D_f = \{x | x \neq 3, x \in \mathbb{R}\} D f = { x ∣ x = 3 , x ∈ R }
Rational functions can be simplified by finding common factors in the numerator and denominator.
Simplify: f ( x ) = 6 x 2 3 x f(x) = \frac{6x^2}{3x} f ( x ) = 3 x 6 x 2
Simplify: f ( x ) = x 2 − 4 x − 2 f(x) = \frac{x^2 - 4}{x - 2} f ( x ) = x − 2 x 2 − 4
Note: x ≠ 2 x \neq 2 x = 2 (from the original domain)
Just like regular fractions, we need to find a common denominator first!
Example: 2 x + 3 x + 1 \frac{2}{x} + \frac{3}{x + 1} x 2 + x + 1 3
Multiply numerator with numerator, denominator with denominator:
Example: x + 1 x × 2 x x − 1 \frac{x + 1}{x} \times \frac{2x}{x - 1} x x + 1 × x − 1 2 x
Remember, dividing means multiplying by the reciprocal:
Determine the domain of f ( x ) = x + 3 x 2 − 9 f(x) = \frac{x + 3}{x^2 - 9} f ( x ) = x 2 − 9 x + 3
Simplify f ( x ) = x 2 − 1 x + 1 f(x) = \frac{x^2 - 1}{x + 1} f ( x ) = x + 1 x 2 − 1
Calculate 1 x − 1 − 2 x + 1 \frac{1}{x - 1} - \frac{2}{x + 1} x − 1 1 − x + 1 2
A car travels 300 km 300 \text{ km} 300 km . If the average speed is v km/h v \text{ km/h} v km/h , write the travel time function in terms of v v v .
Domain: D f = { x ∣ x ≠ − 3 , x ≠ 3 , x ∈ R } D_f = \{x | x \neq -3, x \neq 3, x \in \mathbb{R}\} D f = { x ∣ x = − 3 , x = 3 , x ∈ R }
With the condition x ≠ − 1 x \neq -1 x = − 1