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Chapter 6 Rational Expressions, Functions, and Equations § 6.1 Rational Expressions and Functions: Multiplying and Dividing Rational Expressions EXAMPLE The rational function 130 x f x 100 x models the cost, f (x) in millions of dollars, to inoculate x% of the population against a particular strain of flu. The graph of the rational function is shown. Use the function’s equation to solve the following problem. Find and interpret f (60). Identify your solution as a point on the graph. Blitzer, Intermediate Algebra, 4e – Slide #3 Rational Expressions CONTINUED 1000 900 800 700 600 500 400 300 200 100 0 0 20 40 60 80 Blitzer, Intermediate Algebra, 4e – Slide #4 100 Rational Expressions CONTINUED SOLUTION We use substitution to evaluate a rational function, just as we did to evaluate other functions in Chapter 2. f x 130 x 100 x This is the given rational function. 13060 f 60 100 60 Replace each occurrence of x with 60. 7800 195 40 Perform the indicated operations. Blitzer, Intermediate Algebra, 4e – Slide #5 Rational Expressions CONTINUED Thus, f (60) = 195. This means that the cost to inoculate 60% of the population against a particular strain of the flu is $195 million. The figure below illustrates the solution by the point (60,195) on the graph of the rational function. 1000 900 800 700 600 (60,195) 500 400 300 200 100 0 0 20 40 60 80 Blitzer, Intermediate Algebra, 4e – Slide #6 100 Rational Expressions - Domain EXAMPLE Find the domain of f if f x 3x . 2 x 13x 36 SOLUTION The domain of f is the set of all real numbers except those for which the denominator is zero. We can identify such numbers by setting the denominator equal to zero and solving for x. x 2 13 x 36 0 Set the denominator equal to 0. x 4x 9 0 Factor. Blitzer, Intermediate Algebra, 4e – Slide #7 Rational Expressions - Domain CONTINUED x4 0 x4 or x 9 0 x 9 Set each factor equal to 0. Solve the resulting equations. Because 4 and 9 make the denominator zero, these are the values to exclude. Thus, Domain of f x | x is a real number and x 4 and x 9 or Domain of f - ,4 4,9 9, . Blitzer, Intermediate Algebra, 4e – Slide #8 Rational Expressions Asymptotes Vertical Asymptotes A vertical line that the graph of a function approaches, but does not touch. Horizontal Asymptotes A horizontal line that the graph of a function approaches as x gets very large or very small. The graph of a function may touch/cross its horizontal asymptote. Simplifying Rational Expressions 1) Factor the numerator and the denominator completely. 2) Divide both the numerator and the denominator by any common factors. Blitzer, Intermediate Algebra, 4e – Slide #9 Simplifying Rational Expressions EXAMPLE Simplify: x 1 . 2 x 2x 3 SOLUTION x 1 1 x 1 x 2 2 x 3 x 3x 1 1 x 1 x 3x 1 1 x3 Factor the numerator and denominator. Divide out the common factor, x + 1. Simplify. Blitzer, Intermediate Algebra, 4e – Slide #10 Simplifying Rational Expressions EXAMPLE 2 x 7 x 12 Simplify: . 2 9 x SOLUTION x 2 7 x 12 x 3x 4 2 3 x3 x 9 x Factor the numerator and denominator. x 3x 4 3 x 1 3 x Rewrite 3 – x as (-1)(-3 + x). x 3x 4 3 x 1x 3 Rewrite -3 + x as x – 3. Blitzer, Intermediate Algebra, 4e – Slide #11 Simplifying Rational Expressions CONTINUED x 3x 4 3 x 1x 3 x 4 3 x 1 Divide out the common factor, x – 3. Simplify. Blitzer, Intermediate Algebra, 4e – Slide #12 Multiplying Rational Expressions Multiplying Rational Expressions 1) Factor all numerators and denominators completely. 2) Divide numerators and denominators by common factors. 3) Multiply the remaining factors in the numerators and multiply the remaining factors in the denominators. Blitzer, Intermediate Algebra, 4e – Slide #13 Multiplying Rational Expressions EXAMPLE 2 x 2 3xy 2 y 2 3x 2 2 xy y 2 Multiply: 2 . 2 2 2 3x 4 xy y x xy 6 y SOLUTION 2 x 2 3xy 2 y 2 3x 2 2 xy y 2 2 2 2 3x 4 xy y x xy 6 y 2 2 x y x 2 y 3x y x y 3x y x y x 2 y x 3 y 2 x y x 2 y 3x y x y 3x y x y x 2 y x 3 y This is the original expression. Factor the numerators and denominators completely. Divide numerators and denominators by common factors. Blitzer, Intermediate Algebra, 4e – Slide #14 Multiplying Rational Expressions CONTINUED 2 x y 3x y 3x y x 3 y Multiply the remaining factors in the numerators and in the denominators. Blitzer, Intermediate Algebra, 4e – Slide #15 Dividing Rational Expressions Simplifying Rational Expressions with Opposite Factors in the Numerator and Denominator The quotient of two polynomials that have opposite signs and are additive inverses is -1. Dividing Rational Expressions If P, Q, R, and S are polynomials, where Q 0, R 0, and S 0, then P R P S PS . Q S Q R QR R Change division to multiplication. Replace Swith its reciprocal by interchanging its numerator and denominator. Blitzer, Intermediate Algebra, 4e – Slide #16 Multiplying Rational Expressions EXAMPLE 8y 2 3 y Multiply: 2 2 . y 9 4y y SOLUTION 8y 2 3 y 2 2 y 9 4y y This is the original expression. 24 y 1 3 y y 3 y 3 y4 y 1 24 y 1 3 y y 3 y 3 y4 y 1 (-1) Factor the numerators and denominators completely. Divide numerators and denominators by common factors. Because 3 – y and y -3 are opposites, their quotient is -1. Blitzer, Intermediate Algebra, 4e – Slide #17 Multiplying Rational Expressions CONTINUED 2 y 3y or 2 y 3y Multiply the remaining factors in the numerators and in the denominators. Blitzer, Intermediate Algebra, 4e – Slide #18 Dividing Rational Expressions EXAMPLE xy y 2 2 x 2 xy 3 y 2 Divide: 2 2 . 2 x 2 x 1 2 x 5 xy 3 y SOLUTION xy y 2 2 x 2 xy 3 y 2 2 2 x 2 x 1 2 x 5 xy 3 y 2 This is the original expression. xy y 2 2 x 2 5xy 3 y 2 2 2 x 2 x 1 2 x xy 3 y 2 Invert the divisor and multiply. y x y 2 x 3 y x y 2 x 1 2 x 3 y x y Factor. Blitzer, Intermediate Algebra, 4e – Slide #19 Dividing Rational Expressions CONTINUED y x y 2 x 3 y x y 2 x 1 2 x 3 y x y y x y x 12 Divide numerators and denominators by common factors. Multiply the remaining factors in the numerators and in the denominators. Blitzer, Intermediate Algebra, 4e – Slide #20